Med assess. Ch.17

Pathogens, diseases, and treatments

PATHOGENS, DISEASES, AND TREATMENTS Most microorganisms do not cause disease at all. Some that potentially can cause disease are readily dealt with by the body's defenses discussed earlier in the chapter. However, some microorganisms cause infectious disease even in a healthy person. Worldwide, infectious diseases are responsible for more than 11 million deaths each year. Table 17.2 provides data on some of the more deadly infectious diseases. The pathogens that cause infectious diseases include bacteria, viruses, fungi, protozoa, and parasitic worms (Figure 17.5). Infections can occur almost anywhere in or on the body. Examples of common infections are bronchitis, which is an infection of the airways (bronchi); meningitis, infection of the tissue surrounding the brain and spinal cord; and conjunctivitis, infection of the layer of cells surrounding the eyes. VITAL STATISTICS Table 17.2 Estimated Annual Deaths Due to Top Infectious Diseases Worldwide DISEASE DEATHS Tuberculosis 1.5 million Vaccine-preventable diseases 1.5 million HIV/AIDS 1.2 million Pneumonia in children 922,000 Diarrheal diseases in children 760,000 Neglected tropical diseases* 534,000 Malaria 438,000 *A group of parasitic and bacterial infections primarily affecting low-income populations in developing countries, especially tropical areas; they include trachoma, Hansen's disease (leprosy), dengue fever, Guinea worm, schistosomiasis, and trypanosomiasis (sleeping sickness). sources: World Health Organization. 2016. World Health Statistics 2016: Monitoring Health for the SDGs, Sustainable Development Goals. Geneva: World Health Organization; World Health Organization. 2015. Fact Sheet: Pneumonia (http://www.who.int/mediacentre/factsheets/fs331/en); World Health Organization. 2015. Global Health Observatory Data: Immunization (http://www.who.int/gho/immunization/en); World Health Organization. 2013. Fact Sheet: Diarrhoeal Disease (http://www.who.int/mediacentre/factsheets/fs330/en/); Centers for Disease Control and Prevention. 2016. Neglected Tropic Diseases (http://www.cdc.gov/globalhealth/ntd/fastfacts.html). Table lists pathogens and the infectious diseases associated with them.[D] FIGURE 17.5 Pathogens and associated infectious diseases. Bacteria The most abundant living things on earth are bacteria, which are single-celled organisms that usually reproduce by splitting in two to create a pair of identical cells. Bacteria are Page 483often classified by their shape: bacilli (rod-shaped), cocci (spherical), spirochete (spiral-shaped), or vibrios (comma-shaped). Many species of bacteria are beneficial, but some are pathogenic, causing disease in their hosts. About 100 species of bacteria can cause disease in humans. We harbor both helpful and harmful bacteria on our skin and in our gastrointestinal and reproductive tracts. The human colon contains helpful bacteria that produce certain vitamins and help digest nutrients. Helpful bacteria also keep harmful bacteria in check by competing for food and resources and secreting substances toxic to pathogenic bacteria. For example, Lactobacillus acidophilus resides in the vagina and produces chemicals that kill yeast and bacteria that cause vaginal infections. Pathogenic bacteria cause disease when they release toxins or grow in tissues that are normally sterile, such as the bladder. They can enter the body through a cut in the skin, an insect bite, contaminated food or drink, sexual activity, or any of several other means. They usually cause illness by invading body cells or by producing toxins. Pneumonia Inflammation of the lungs, called pneumonia, may be caused by infection with bacteria, viruses, or fungi, or by contact with chemical toxins or irritants. Pneumonia can be serious if the alveoli (air sacs) become clogged with fluid, thus preventing oxygen from reaching the bloodstream. Pneumonia often follows another illness, such as a cold or the flu, but the symptoms are typically more severe—fever, chills, shortness of breath, increased mucus production, and cough. Pneumonia is one of the 10 leading causes of death for Americans; people most at risk for severe infection include those under age 2 or over age 75 and those with chronic health problems such as heart disease, asthma, or HIV. Worldwide, pneumonia is the leading cause of death for children under 5 years of age. The most common cause of bacterial pneumonia is Streptococcus pneumoniae, or pneumococcus. A vaccine is available for pneumococcal pneumonia and is recommended for all adults age 65 and over and others at risk. Other bacteria that may cause pneumonia include Haemophilus influenzae, Chlamydia pneumoniae, and Mycoplasma pneumoniae. Mycoplasma is a very small bacterium; M. pneumoniae causes a mild form of pneumonia often called "walking pneumonia." Outbreaks of infection with mycoplasmas are relatively common among young adults, especially in crowded settings such as dormitories. Bacterial pneumonia can be treated with antibiotics, which we will discuss shortly. Meningitis Inflammation of the meninges, the protective membranes covering the brain and spinal cord, is called meningitis. Inflammation is usually caused by infection of the fluid surrounding the brain. The infection can be caused by a bacterium, virus, fungus, or parasite. Most cases of meningitis are viral; they are usually mild and resolve without medical intervention. Bacterial meningitis, however, can be life threatening and requires immediate treatment with antibiotics. Symptoms of meningitis include fever, a severe headache, stiff neck, sensitivity to light, and confusion. Symptoms can appear quickly or over a few days. Immediate treatment is needed because death can occur within hours. Before the 1990s, Haemophilus influenzae type b (Hib) was the leading cause of bacterial meningitis, but routine vaccination of children has reduced the occurrence of Hib meningitis. Today Neisseria meningitidis (meningococcus) and Streptococcus pneumoniae (pneumococcus) are the leading causes of bacterial meningitis, particularly in adolescents and young adults (see the box "Meningococcal Meningitis and College Students"). WELLNESS ON CAMPUS: Meningococcal Meningitis and College Students When meningitis is caused by the bacterium Neisseria meningitidis, it is called meningococcal meningitis. Although rates of this disease have been declining since the 1990s, it still causes deaths. The disease is spread by the exchange of respiratory and throat secretions (as in coughing and sneezing) and through person-to-person contact (as in kissing); it can readily spread among people living in close quarters. In the United States, nearly 30% of meningitis cases occur in adolescents and young adults, and there is an increased risk in college students. Lifestyle factors, such as crowded living situations, irregular sleep patterns, sharing of personal items, bar patronage, and smoking (active or passive) all increase the risk for meningococcal meningitis. A vaccine is available that protects against four of the five most common strains of meningitis. Because rates of the disease rise beginning in early adolescence and peak between ages 15 and 20, the CDC and the American College Health Association recommend vaccination for all unvaccinated first-year college students living in dormitories, and some schools require it. Visit www.immunize.org/laws/menin.asp to find out more about specific state mandates. Meningococcal meningitis is treated with antibiotics, and often those who have direct contact with the patient, such as housemates, roommates, and romantic partners, are also given antibiotics. Besides being vaccinated, you can protect yourself against meningococcal meningitis by getting enough sleep, not smoking and not being around tobacco smoke, and avoiding people who are sick. sources: Centers for Disease Control and Prevention. 2016. Meningitis (http://www.cdc.gov/meningitis); Vaccines.gov. n.d. College & Young Adults (http://www.vaccines.gov/who_and_when/college/); American College Health Association. 2015. Meningitis on Campus: Know Your Risk (http://www.acha.org/ACHA/Resources/Topics/Meningitis.aspx). In the United States, between 1400 and 3000 cases of meningitis and between 150 and 300 deaths are reported each year, although the actual numbers are probably higher. The disease is fatal in about 10% of cases, and about 10-20% of people who recover have permanent disabilities, including brain damage, seizures, and hearing loss. Worldwide, meningitis kills about 170,000 people each year, particularly in the so-called meningitis belt in sub-Saharan Africa. Page 484 A vaccine is available, but it is not effective against all strains of meningitis-causing bacteria. The CDC recommends routine vaccination of children aged 11-18 years, previously unvaccinated adolescents at high school entry, and first-year college students who live in dormitories. Strep Throat and Other Streptococcal Infections Streptococcus is the genus name of a group of bacteria that cause several diseases in humans. Streptococcal pharyngitis, or strep throat, is characterized by a red, sore throat with white patches on the tonsils, swollen lymph nodes, fever, and headache. Typically the streptococcus bacterium is spread from an infected individual through close contact via respiratory droplets that are released when the infected person sneezes or coughs. If left untreated, strep throat can develop into the more serious rheumatic fever (see Chapter 15). Other streptococcal infections include scarlatina (scarlet fever), characterized by a sore throat, fever, bright red tongue, and rash over the upper body; impetigo, a superficial skin infection most Page 485common among children; and erysipelas, which causes inflammation of skin and underlying tissues. Streptococcal infections are treated with antibiotics. If you are have a streptococcal infection, you can minimize the risk of spreading it by washing your hands well, especially before preparing foods, and by staying home until at least 24 hours after starting a course of antibiotics. A particularly virulent type of streptococcus can invade the bloodstream, spread to other parts of the body, and produce dangerous systemic illness. It can also cause a serious but rare infection of the deeper layers of the skin, a condition called necrotizing fasciitis or "flesh-eating strep." This dangerous infection is characterized by tissue death and is treated with antibiotics and removal of the infected tissue or limb. Other species of streptococci are implicated in pneumonia, endocarditis (infection of the heart lining and valves), and serious infections in pregnant women and newborns. Toxic Shock Syndrome and Other Staphylococcal Infections Staphylococcus bacteria are commonly found on the skin and in the nasal passages of healthy people. For example, Staphylococcus aureus occurs on 15-40% of people who show no signs of disease. Occasionally, however, staphylococci enter the body and cause an infection, ranging from minor skin infections such as boils to very serious conditions such as blood infections and pneumonia. The risk of a serious staph infection is higher in persons with certain medical conditions, including people suffering from malnutrition, alcoholism, intravenous drug use, diabetes, or kidney failure. Staphylococcus aureus is responsible for many cases of toxic shock syndrome (TSS). The bacteria produce a toxin that causes a massive proliferation of T cells that can bind to it. The staphylococcus toxins are referred to as "super antigens" because the massive proliferation of T cells results in excessive production of pro-inflammatory cytokines. This "cytokine storm" causes tissue damage, widespread coagulation of blood in the blood vessels, and ultimately organ failure. TSS was first diagnosed in women using highly absorbent tampons, which appear to allow the growth of staphylococci; however, about half of all cases occur in men and in women not using tampons. (See Chapter 6 for information about toxic shock syndrome as it relates to contraception.) A serious antibiotic-resistant infection is caused by a staphylococcus bacterium known as methicillin-resistant Staphylococcus aureus (MRSA). Many cases of MRSA infection occur in hospitalized patients, and these infections tend to be severe. They include infections of surgical wounds, pneumonia, urinary tract infections, and blood infections. MRSA is also common in the community (outside hospital settings), where it usually infects the skin, causing painful lesions that resemble infected spider bites. The spread of MRSA infection is associated with places where people Page 486are in close contact with one another, such as locker rooms and playing fields; athletes are particularly at risk and should keep any cuts or abrasions covered. The best defense against MRSA, whether in health care settings or in the community, is good hygiene and frequent hand washing. Skin infections caused by methicillin-resistant Staphylococcus aureus (MRSA) can be very hard to treat. source: Centers for Disease Control and Prevention/Gregory Moran, M.D. Tuberculosis Caused by the bacterium Mycobacterium tuberculosis, tuberculosis (TB) is a chronic bacterial infection that usually affects the lungs, though it can affect other organs as well. TB is spread via the respiratory route. Symptoms include coughing, fatigue, night sweats, weight loss, and fever. Between 10 and 15 million Americans have been infected with M. tuberculosis and continue to carry it. Only about 10% of people with latent TB infections actually develop an active case of the disease; the immune system usually prevents the disease from becoming active. In the United States, active TB is most common among people infected with HIV, recent immigrants from countries where TB is endemic, and those who live in inner cities. In 2014, a total of 9421 new cases of TB were reported—an incidence of 3.0 cases per 100,000 population. This is the lowest rate since national tracking began in 1953, but the rate of decline has slowed. Many strains of tuberculosis respond to antibiotics, but only over a course of treatment lasting 6-12 months. Failure to complete treatment can lead to relapse and the development of strains of antibiotic-resistant bacteria. Multidrug-resistant TB (MDR TB) is resistant to at least two of the best anti-TB drugs. Extensively drug-resistant TB (XDR TB) is resistant to those drugs as well as some second-line drugs. XDR TB is relatively rare, but patients with this form of TB have fewer and less effective treatment options. TB is a leading cause of death in people with HIV infection. Tick-borne Infections Some diseases are transmitted via insect vectors. Lyme disease is one such infection, and it accounts for more than 25,000 cases per year. It is spread by the bite of a tick that is infected with the spiral-shaped bacterium Borrelia burgdorferi. Ticks acquire the bacterium by ingesting the blood of an infected animal; they can then transmit the microbe to their next host. The deer tick is responsible for transmitting Lyme disease bacteria to humans in the northeastern and north-central United States; on the Pacific Coast, the culprit is the western black-legged tick. Lyme disease has been reported in 48 states, but significant risk of infection is found in only about 100 counties in 10 states located in the northeastern and mid-Atlantic seaboard, the upper north-central region, and parts of northern California. Symptoms of Lyme disease vary but typically occur in three stages. In the first stage, about 80% of victims develop a bull's-eye-shaped red rash expanding from the area of the bite, usually about two weeks after the bite occurs. The second stage occurs weeks to months later in 10-20% of untreated patients; symptoms may involve the nervous and cardiovascular systems and can include impaired coordination, partial facial paralysis, and heart rhythm abnormalities. These symptoms usually disappear on their own within a few weeks. The third stage, which occurs in about half of untreated people, can develop months or years after the tick bite and usually consists of chronic or recurring arthritis. Lyme disease can also cause fetal damage or death at any stage of pregnancy. Lyme disease is treatable at all stages, although arthritis symptoms may not resolve completely. Lyme disease is preventable by avoiding contact with ticks or by removing a tick before it has had the chance to transmit the infection. Rocky Mountain spotted fever and typhus are caused by the rickettsia bacterium and are also transmitted via tick bites. Rocky Mountain spotted fever is characterized by sudden onset of fever, headache, and muscle pain, followed by development of a spotted rash. Ehrlichiosis, another tick-borne disease, typically causes less severe symptoms. Ulcers About 25 million Americans suffer from ulcers, which are sores in the lining of the stomach or the first section of the small intestine (duodenum). Long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs) can contribute to the development of ulcers. However, up to 90% of ulcers are caused by infection with the bacterium Helicobacter pylori. If tests show the presence of H. pylori, antibiotics often cure the infection and the ulcers. Individuals who take NSAIDs and contract ulcers should be tested for H. pylori because they are likely infected and termination of NSAIDs alone will not cure the ulcers. Page 487 Other Bacterial Infections The following are a few of the many other infections caused by bacteria: Tetanus. Also known as lockjaw, tetanus is caused by the bacterium Clostridium tetani, which thrives in deep puncture wounds and produces a deadly toxin. The toxin causes muscular stiffness and spasms, and infection is fatal in about 30% of cases. Due to widespread vaccination, tetanus is rare in the United States. Worldwide, however, more than 200,000 people die from tetanus each year, primarily newborns infected through unsterile cutting of the umbilical cord. Clostridium difficile. Another type of Clostridium bacteria, called Clostridium difficile (C. diff), has joined MRSA as a major emerging threat in American health care settings, particularly hospitals. Considered a health-care-associated infection, C. diff occurs in patients with conditions requiring prolonged use of antibiotics. It causes inflammation of the colon, resulting in diarrhea, fever, and nausea. Bacterial spores can live outside the body for long periods and can be found on objects like medical equipment, bathroom fixtures, and bed linens, as well as on people's hands. Most cases of C. diff infection respond to a new class of antibiotics that selectively kills C. diff without affecting the many bacterial species that populate the normal, healthy intestine. Nevertheless, about 14,000 people die each year from this infection. The CDC recommends that doctors, nurses, and other health care providers wash their hands frequently to reduce the spread of the bacterium. Pertussis. Also known as whooping cough, pertussis is a highly contagious respiratory illness caused by the bacterium Bordetella pertussis. Pertussis is characterized by bursts of rapid coughing, followed by a long attempt at inhalation that is often accompanied by a high-pitched whoop; symptoms may persist for two to eight weeks. The number of pertussis cases in the United States has risen over the past two decades, up to 18,000 cases in 2015. Those at high risk include infants and children who are too young to be fully vaccinated and those who have not completed the primary vaccination series. Adolescents and adults become susceptible when immunity from vaccination wanes, so a booster shot is recommended at 11-12 years or during adolescence and thereafter every 10 years. Adults account for about 28% of whooping cough cases. Urinary tract infections (UTIs). Infection of the bladder and urethra is most common among sexually active women, but UTIs can occur in anyone. The bacterium Escherichia coli (E. coli) is the most common infectious agent, responsible for about 80% of all UTIs. Infection most often occurs when bacteria from the digestive tract that live on the skin around the anus get pushed toward the opening of the urethra during sexual intercourse; the bacteria then travel up the urethra and into the bladder. Urinating before and after intercourse may help prevent UTIs. Other illnesses caused by bacteria include foodborne illness, discussed in Chapter 12, and sexually transmitted infections, discussed in Chapter 18. Antibiotic Treatments The body's immune system can fight off many, if not most, bacterial infections. However, even as the body musters its defenses, some bacteria can cause a great deal of damage. Antibiotics can help the body deal with these infections. Antibiotics are drugs that either inhibit the growth of bacteria or kill them. Some antibiotics have also been developed to inhibit fungi and protozoa, but antibiotics with antibacterial activity are the most widely used, and these are the ones we will discuss here. Actions of Antibiotics Antibiotics are both naturally occurring and synthetic substances. Antibacterial antibiotics are categorized based on their mechanism of action. The majority of antibiotics inhibit the synthesis of the bacterial cell wall. The second largest group interferes with the production of bacterial proteins. A third class prevents the replication of the bacterial DNA. QUICK STATS Each year, at least 2 million Americans become infected with antibiotic-resistant bacteria, and 23,000 die as a direct result of these infections. —Centers for Disease Control and Prevention, 2016a Antibiotic Resistance Antibiotics have saved millions of lives. However, their overuse and misuse has led to the emergence of antibiotic-resistant bacteria. A bacterium can become resistant from a chance genetic mutation or through the transfer of genetic material from one bacterium to another. When exposed to antibiotics, resistant bacteria can grow and flourish while the antibiotic-sensitive bacteria die off. Eventually an entire colony of bacteria can develop resistance to one or more antibiotics and become very difficult to treat. Antibiotic-resistant strains of many common bacteria have developed, including strains of gonorrhea, salmonella, and tuberculosis. Antibiotic resistance is a major factor contributing to the rise in problematic infectious diseases. The more often bacteria encounter antibiotics, the more likely they are to develop resistance. Resistance is promoted when people fail to take the full course of an antibiotic or when they inappropriately take antibiotics for viral infections. Another possible source of resistance is the use of antibiotics in agriculture, which is estimated to account for 50-80% of the 25,000 tons of antibiotics used annually in the United States. At least four species of antibiotic-resistant bacteria have been transmitted from food animals to humans, leading the U.S. Food and Drug Administration (FDA) to issue guidelines to the pharmaceutical industry regarding the use of new antibiotics in food-producing animals. Recent studies show that antibiotics cause long-term changes to the protective bacteria in our bodies and may cause an increased susceptibility to infections and disease. Page 488 You can help prevent the development of antibiotic-resistant strains of bacteria by using antibiotics properly: Don't ask your doctor for an antibiotic every time you get sick. Antibiotics are helpful for bacterial infections but are ineffective against viruses. Use antibiotics as directed, and finish the full course of medication even if you begin to feel better. Doing so helps ensure that all targeted bacteria are killed off. Never take an antibiotic without a prescription. If you take an antibiotic for a viral infection, your illness will not improve. Viruses A virus is a microscopic organism consisting of genetic material covered by a protein coat. Visible only with an electron (high-magnification) microscope, viruses are about one one-hundredth of the size of a typical bacterium. Viruses lack the enzymes essential to energy production and protein synthesis in normal animal cells, and they can replicate only inside the cells of another organism. Once a virus is inside the host cell, it sheds its protein covering and exploits the host's cellular machinery to produce more viruses like itself (Figure 17.6). The immune system of the host responds by producing substances called interferons—the first line of defense against viral infection. Interferons block the spread of virus infection to other cells in the body and can stimulate the immune system to kill infected cells. Step-by-step description of how a virus infects and replicates within a cell.[D] FIGURE 17.6 Life cycle of a virus. Most contagious diseases are caused by viruses. Different viruses infect different kinds of cells, and the seriousness of the disease they cause depends greatly on which kind of cell is infected. The viruses that cause colds, for example, attack upper respiratory tract cells, which are constantly replaced; the disease is therefore mild in otherwise healthy individuals. HIV infection, a viral illness that attacks the cells of the immune system, can destroy the body's ability to fight infectious diseases and is thus extremely serious (see Chapter 18). The Common Cold Although generally brief, lasting only one to two weeks, colds are nonetheless irritating and often interfere with your normal activities. A cold may be caused by any of more than 200 viruses that attack the lining of the nasal passages; rhinoviruses and coronaviruses cause a large percentage of all colds among adults. Cold viruses are almost always transmitted by hand-to-hand contact. To lessen your risk of contracting a cold, wash your hands frequently; if you touch someone else, avoid touching your face until after you've washed your hands. Page 489 If you catch a cold, over-the-counter cold remedies may relieve your symptoms, but they do not eliminate the virus (see the box "Preventing and Treating the Common Cold"). Sometimes it is difficult to determine whether your symptoms are due to a virus (as in colds, flu, and some sinus infections), a bacterium (as in other sinus infections), or an allergy, but this information is important for appropriate treatment (Table 17.3). For example, antibiotics will not affect a viral infection but will help treat a bacterial sinus infection. CRITICAL CONSUMER: Preventing and Treating the Common Cold Prevention Colds are usually spread by hand-to-hand contact with another person or with objects such as doorknobs and remote controls that an infected person has handled. The best way to avoid transmission is to wash your hands frequently with warm water and soap. Keeping your immune system strong is another good prevention strategy (see "Supporting Your Immune System" later in the chapter). Home Treatments Get extra rest. It isn't usually necessary to stay home in bed, but you will need to slow down a little from your usual routine to give your body a chance to fight the infection. Drink plenty of liquids to prevent dehydration. Hot liquids such as herbal tea and clear chicken soup will soothe a sore throat and loosen secretions; gargling with slightly salty water may also help. Avoid alcoholic beverages when you have a cold. Use a humidifier to help eliminate nasal stuffiness and soothe inflamed membranes. Use a personal steam inhaler for 20 minutes when you suspect that you have been in contact with someone who has a cold. This treatment can abort a cold in its early stages but is not so effective if the infection is beyond the sore throat stage. © Stockdisc/PunchStock RF Over-the-Counter Treatments Avoid multi-symptom cold remedies. Because these products include drugs to treat symptoms you may not have, you risk experiencing side effects from medications you don't need. It's better to treat each symptom separately: Analgesics—aspirin, acetaminophen (Tylenol), ibuprofen (Advil or Motrin), and naproxen sodium (Aleve)—all help lower fever and relieve muscle aches. Use of aspirin is associated with an increased risk of a serious condition called Reye's syndrome in children and teenagers; for this reason, aspirin should be given only to adults. Decongestants shrink nasal blood vessels, relieving swelling and congestion. However, they may dry out mucous membranes in the throat and make a sore throat worse. Nasal sprays shouldn't be used for more than two or three days to avoid rebound congestion. Cough medicines may be helpful when your cough is nonproductive (not bringing up mucus) or if it disrupts your sleep or work. Expectorants make coughs more productive by increasing the volume of mucus and decreasing its thickness, thereby helping remove irritants from the respiratory airways. Cough suppressants (antitussives) reduce the frequency of coughing. Antihistamines decrease nasal secretions caused by the effects of histamine, so they are much more useful in treating allergies than in treating colds. Many antihistamines can make you drowsy. The jury is still out on whether other remedies, including zinc gluconate lozenges, echinacea, and vitamin C, relieve symptoms or shorten the duration of a cold. Researchers are also studying antiviral drugs that target the most common types of cold viruses. Sometimes a cold leads to a more serious complication, such as bronchitis, pneumonia, or strep throat. If a fever of 102°F or higher persists, or if cold symptoms don't abate after two weeks, see your physician. Table 17.3 Is It a Cold or the Flu? Symptoms Cold Flu Fever Rare Usual; high (100-102°F; occasionally higher, especially in young children); lasts 3-4 days Headache Rare Common General aches, pains Slight Usual; often severe Fatigue, weakness Sometimes Usual; can last up to 2-3 weeks Extreme exhaustion Never Usual; at the beginning of the illness Stuffy nose Common Sometimes Sneezing Usual Sometimes Sore throat Common Sometimes Chest discomfort, cough Mild to moderate; hacking cough Common; can become severe Treatment Antihistamines, decongestants, nonsteroidal anti-inflammatory medicines Antiviral medicines—see your healthcare provider Prevention Wash your hands often with soap and water; avoid close contact with anyone who has a cold Annual vaccination; antiviral medicines—see your healthcare provider Complications Sinus infection; middle ear infection; asthma Bronchitis; pneumonia; can worsen chronic conditions; can be life-threatening (complications are more likely in the elderly, those with chronic conditions, young children, and pregnant women) source: National Institutes of Health. National Institute of Allergy and Infectious Diseases. 2014. Is It a Cold or the Flu? (https://www.niaid.nih.gov/topics/Flu/Documents/sick.pdf). Influenza Commonly called the flu, influenza is an infection of the respiratory tract caused by the influenza virus. (Many people use the term "stomach flu" to describe gastrointestinal illnesses, but these infections are actually caused by organisms other than influenza viruses.) Compared to the common cold, influenza is a more serious illness, usually including a fever and extreme fatigue. Most people who get the flu recover within one to two weeks, but some develop potentially life-threatening Page 490complications, such as pneumonia. The highest rates of infection occur in children. Influenza is highly contagious and is spread via respiratory droplets. The most effective way to prevent the flu is through annual vaccination. The seasonal influenza vaccine consists of killed virus and provides protection against the strains of the virus currently circulating; it is updated each year in response to changes in the virus. The CDC recommends vaccination for all people age 6 months and over. Residents of nursing homes and long-term care facilities, health care workers, and household contacts and caregivers of children up to age 5 years are particularly at risk. In addition to the injected vaccine, a nasal aerosol vaccine is available for healthy children aged 24-59 months and for nonpregnant healthy people aged 5-49 years. Measles, Mumps, and Rubella Three viral childhood illnesses that have decreased in the United States due to effective vaccines are measles, mumps, and rubella. Measles and rubella are generally characterized by rash and fever. Measles can occasionally cause more severe illness, including liver or brain infection or pneumonia. Worldwide, more than 600,000 people die each year from measles. Measles is a highly contagious disease, and before the introduction of vaccines, more than 90% of Americans contracted measles by age 15. Rubella, if it infects a pregnant woman, can be transmitted to a fetus, causing miscarriage, stillbirth, and severe birth defects, including deafness, eye and heart defects, and mental impairment. Mumps generally causes swelling of the parotid (salivary) glands, located just below and in front of the ears. This virus can also cause meningitis and, in males, inflammation of the testes. Chickenpox, Cold Sores, and Other Herpesvirus Infections The herpesviruses are a large group of viruses. Once infected, the host is never free of the virus. The virus lies latent within certain cells and becomes active periodically, producing symptoms. Herpesviruses are particularly dangerous for people with a depressed immune system, as in the case of HIV infection. The family of herpesviruses includes the following: Varicella-zoster virus, which causes chickenpox and shingles. Chickenpox is a highly contagious disease characterized by an itchy rash made up of small blisters. The infection is usually mild, although complications are more likely to occur in young infants and adults. After the rash resolves, the virus becomes latent, living in sensory nerves. Many years later, the virus may reactivate and cause shingles. Symptoms of shingles include pain in the affected nerves and a rash on the skin that follows the pattern of the nerve pathways (often a band over the ribs on one side of the body). A vaccine is available that prevents chickenpox in the majority of cases and results in milder illness if the disease does occur. It is recommended for children over 12 months of age, unvaccinated adolescents, and adults who have not had chickenpox. A shingles vaccine is recommended for people age 60 and older regardless of their history of chickenpox or shingles.Page 491 Herpes simplex virus (HSV) types 1 and 2, which cause cold sores and the sexually transmitted infection herpes (see Chapter 18). Herpes infections are characterized by small, painful ulcers in the area around the mouth or genitals, at the site where a person first contracts the virus. Following the initial infection, HSV becomes latent and may reactivate again and again over time. Many infected people do not know they are infected, and the virus can be transmitted even when sores are not apparent. Antiviral medications are available to prevent recurrences of genital herpes. Epstein-Barr virus (EBV), which causes infectious mononucleosis. Mono, as it is commonly called, is characterized by fever, sore throat, swollen lymph nodes, and fatigue. It is usually spread by intimate contact with the saliva of an infected person—hence the nickname "kissing disease." Mono most often affects adolescents and young adults. EBV can reactivate throughout life with a recurrence of symptoms. In a few people, especially those with HIV infection, EBV is associated with the development of cancers of the lymph system. Two herpesviruses that can cause severe infections in people with AIDS or who have a suppressed immune system are cytomegalovirus (CMV), which infects the lungs, brain, colon, and eyes, and human herpesvirus 8 (HHV-8), which has been linked to Kaposi's sarcoma, a cancer of the connective tissue. Viral Encephalitis HSV type 1 is a possible cause of viral encephalitis—inflammation of brain tissue due to a viral infection. Other possible causes include HIV and several mosquito-borne viruses, including Japanese encephalitis virus, equine encephalomyelitis virus, and West Nile virus. Mild cases of encephalitis may cause fever, headache, nausea, and lethargy; severe cases are characterized by memory loss, delirium, diminished speech function, and seizures, and they may result in permanent brain damage or death. Viral Hepatitis Viral hepatitis is a term used to describe several different infections that cause inflammation of the liver. Hepatitis is usually caused by one of the three most common hepatitis viruses: Hepatitis A virus (HAV) causes the mildest form of the disease and is usually transmitted by food or water contaminated by sewage or an infected person. Hepatitis B virus (HBV) is usually transmitted sexually (see Chapter 18). Hepatitis C virus (HCV) can also be transmitted sexually, but it is much more commonly passed through direct contact with infected blood via injection drug use or—prior to the development of screening tests—blood transfusions. HBV and, to a lesser extent, HCV can also be passed from a pregnant woman to her child. There are effective vaccines for hepatitis A and B, but more than 150,000 new cases of hepatitis occur in the United States each year. Symptoms of acute hepatitis infection can include fatigue, jaundice, abdominal pain, loss of appetite, nausea, and diarrhea. Most people recover from hepatitis A within a month or so. However, 5-10% of people infected with HBV and 85-90% of people infected with HCV become chronic carriers of the virus, capable of infecting others for the rest of their lives. Some chronic carriers remain asymptomatic, while others slowly develop chronic liver disease, cirrhosis, or liver cancer. An estimated 4 million Americans and 500 million people worldwide may be chronic carriers of hepatitis. Each year in the United States, HBV and HCV are responsible for more than 15,000 deaths. The extent of HCV infection has been recognized only recently, and most infected people are unaware of their condition. To ensure proper treatment and prevention, testing for HCV may be recommended for people at risk, including people who have injected drugs (even once); who received a blood transfusion or a donated organ prior to July 1992; who have engaged in high-risk sexual behavior; or who have had body piercing, tattoos, or acupuncture involving unsterile equipment (see the box "Tattoos and Body Piercing"). Antiviral drugs are available to treat chronic hepatitis, but they are not completely effective and may have significant side effects. CRITICAL CONSUMER: Tattoos and Body Piercing Because tattooing and body piercing involve the use of needles, they carry health risks. Tattoos are permanent marks applied with an electrically powered instrument that injects dye into the second layer of the skin. A tattoo typically takes a week or two to heal and should be protected from sun exposure until then. In piercing, the artist pushes a needle through the skin; a piece of jewelry holds the piercing open. Healing time varies depending on the site of the piercing and other factors. Potential Health Complications Infection. There is a risk of transmission of blood-borne infectious agents, such as hepatitis, if instruments are not sterilized properly. In 2006, the CDC reported an outbreak of methicillin-resistant Staphylococcus aureus among customers of tattoo parlors in several states. Due to the potential risks of infection, people currently cannot donate blood for 12 months following application of body art, including tattoos and some body piercings. People with heart valve problems should check with a physician prior to body piercing to determine if they should take antibiotics before the procedure. Allergic reactions. Some people may be allergic to pigments used in tattooing or to metals used in body-piercing jewelry. All jewelry should be made of noncorrosive materials such as stainless steel or titanium; avoid jewelry that contains nickel. Nodules and scars. Some people may develop granulomas (nodules) or keloids (a type of scar) following tattooing or body piercing. Choosing a Body Artist and Studio A body art studio should be clean and have an autoclave for sterilizing instruments. Needles should be sterilized and disposable; piercing guns should not be used because they cannot be sterilized adequately. The body artist should wear disposable latex gloves throughout the procedure. Ask if the studio and/or artist are members of the Alliance of Professional Tattooists (www.safe-tattoos.com) or the Association of Professional Piercers (www.safepiercing.org); these organizations have developed infection control and other guidelines for their members to follow. © blueskyimage/123RF The National Institute for Occupational Safety and Health issues guidelines designed to protect both tattoo artists and customers from blood-borne infections. They cover such practices as washing hands, changing gloves, disinfecting surfaces, cleaning and sterilizing tools and equipment, and receiving training. Regulations and recommended practices vary from state to state; for more information, contact your state's public health department. Poliomyelitis An infectious viral disease that affects the nervous system, poliomyelitis (polio) can cause irreversible paralysis and death in some affected individuals. As with other vaccine-preventable diseases, the incidence of polio declined dramatically in the United States following the introduction of the vaccine. North and South America are now considered free of the disease. It has also been eradicated from most countries of the world and remains endemic only in Afghanistan and Pakistan. Rabies Caused by a rhabdovirus that infects both humans and animals, the rabies virus infects the nervous system and is fatal if untreated. Humans become infected by rabies when they are bitten by a rabies-infected animal. U.S. rabies-related deaths among humans declined dramatically during the 20th century due to the widespread vaccination of domestic animals and the development of a highly effective vaccine regimen that provides immunity following Page 492exposure (post-exposure prophylaxis, or PEP). PEP consists of one dose of immunoglobulin and five doses of rabies vaccine over a 28-day period. Human Papillomavirus (HPV) The more than 200 types of HPV cause a variety of warts (noncancerous skin tumors), including common warts on the hands, plantar warts on the soles of the feet, and genital warts around the genitalia. Depending on their location, warts may be removed using over-the-counter preparations or professional methods such as laser surgery or cryosurgery. Because HPV infection is chronic, warts can reappear despite treatment. As described in Chapter 16, HPV causes the majority of cases of cervical cancer. Three vaccines are available, targeting the types of HPV linked to the majority of HPV-caused cancers. Vaccination is recommended for all girls and boys aged 11 or 12; women can get the HPV vaccine through age 26 and men, through age 21. HPV is discussed in detail in Chapters 16 and 18. An analysis of a four-year clinical trial showed that an HPV vaccine is 100% effective in preventing the abnormal proliferation of cervical cells that precedes the development of cervical cancer. QUICK STATS Only 3 countries—Afghanistan, Nigeria, and Pakistan—reported cases of polio in 2016, down from more than 125 countries in 1988. —World Health Organization, 2016 Treating Viral Illnesses Antiviral drugs typically work by interfering with some part of the viral life cycle; for example, they may prevent a virus from entering body cells or from successfully reproducing within cells. Antivirals are currently available to fight infections Page 493caused by HIV, influenza, herpes simplex, varicella-zoster, HBV, and HCV. Most other viral diseases must simply run their course. Fungi A fungus is an organism that reproduces by spores and feeds on organic matter. Only about 50 of the many thousands of species of fungi cause disease in humans, and these diseases are usually restricted to the skin, mucous membranes, and lungs. Some fungal diseases are extremely difficult to treat because spores are an especially resistant dormant stage of the organism. Candida albicans is a common fungus found naturally in the vagina of most women. When excessive growth occurs, the result is itching and discomfort, commonly known as a yeast infection. Factors that increase the growth of C. albicans include the use of antibiotics, clothing that keeps the vaginal area excessively warm and moist, pregnancy, oral contraceptive use, and certain diseases, including diabetes and HIV infection. The most common symptom is usually a thick white or yellowish discharge. Prescription and over-the-counter treatments are available. Women should not self-treat unless they are certain from a past medical diagnosis that they have a yeast infection. (Misdiagnosis could mean that a different and more severe infection goes untreated.) C. albicans overgrowth can occur in other areas of the body, especially in the mouths of infants (a condition known as thrush). Other common fungal conditions, including athlete's foot, jock itch, and ringworm, affect the skin. These three conditions are usually mild and easy to cure. Fungi can also cause systemic diseases that are severe, life threatening, and extremely difficult to treat. Histoplasmosis, or valley fever, causes pulmonary and sometimes systemic disease and is most common in the Mississippi and Ohio River Valleys. Coccidioidomycosis is also known as valley fever because it is most frequent in the San Joaquin Valley of California. Fungal infections can be especially deadly in people with impaired immune systems. Protozoa Protozoa are single-celled organisms that can cause a range of diseases in humans. Millions of people in developing countries suffer from protozoal infections. Malaria, caused by a parasitic protozoan of the genus Plasmodium, is characterized by recurrent attacks of severe flulike symptoms (chills, fever, headache, nausea, and vomiting) and may cause anemia. The protozoan is injected into the bloodstream via a mosquito bite. Although now relatively rare in the United States, malaria is a major killer worldwide; each year there are 350-500 million new cases of malaria globally and more than 1 million deaths, mostly among infants and children. Drugs are available to prevent and treat malaria, but in the poorest, most remote areas, conditions make it difficult to distribute drugs. Drug-resistant strains of malaria have emerged, requiring new medicines. Even seemingly pristine mountain streams can be contaminated with the protozoan Giardia lamblia, as many unwary hikers and campers have discovered. Symptoms of giardiasis include abdominal pain, bloating, and diarrhea. © Vitaliy Mateha/123RF Giardiasis is caused by Giardia lamblia, a single-celled parasite that lives in the intestines of humans and animals. Giardiasis is characterized by nausea, diarrhea, bloating, and abdominal cramps, and it is among the most common waterborne diseases in the United States. People may become infected with Giardia if they consume contaminated food or water or pick up the parasite from the contaminated surface of an object such as a bathroom fixture, diaper pail, or toy. People at risk include child care workers, children who attend day care, international travelers, and hikers and campers who drink untreated water. Giardiasis is rarely serious and can be treated with prescription medications. Other protozoal infections include the following: Trichomoniasis, a common vaginal infection. Although usually mild and treatable, trich may increase the risk of HIV transmission (see Chapter 18). Trypanosomiasis (African sleeping sickness), which is transmitted through the bite of an infected tsetse fly and causes extreme fatigue, fever, rash, severe headache, central nervous system damage, and death. Amoebic dysentery, a severe form of amebiasis—infection of the intestines with the parasite Entamoeba histolytica. It is characterized by bloody diarrhea, stomach pain, and fever. Page 494 Parasitic Worms The parasitic worms are the largest organisms that can enter the body to cause infection. Worms, including intestinal parasites such as the tapeworm and hookworm, cause a variety of relatively mild infections. Pinworm, the most common worm infection in the United States, primarily affects young children. Smaller worms known as flukes infect organs such as the liver and lungs and, in large numbers, can be deadly. Worm infections generally originate from contaminated food or drink and can be prevented by careful attention to hygiene. Emerging Infectious Diseases Emerging infectious diseases are infections whose incidence in humans has increased or threatens to increase in the near future. They include both known diseases that have experienced a resurgence, such as tuberculosis and cholera, and diseases that were previously unknown or confined to specific areas, such as the Zika and Ebola viruses. Selected Infections of Concern Although the chances of the average American contracting an exotic infection are very low, emerging infections are a concern to public health officials and represent a challenge to all nations in the future. Zika Disease Zika virus is transmitted by several species of Aedes mosquitoes; it can also spread through sex with a man infected with Zika, and it can pass from a pregnant woman to her fetus. The virus was discovered in Uganda in 1947 and arrived in Southeast Asia shortly thereafter. By 2014, it had spread across the Pacific Islands and into the Americas; as of mid-2016, active virus transmission has been reported throughout most of South and Central America, the Caribbean (including Puerto Rico), Mexico, and parts of Florida. In most people, Zika symptoms are very mild, lasting a week or less, and may include fever, rash, and joint pain. However, a small proportion of people infected with Zika develop a neurological condition called Guillain-Barré syndrome, usually characterized by muscle weakness; this condition typically resolves within a few weeks or several months. Of greater concern are the effects of Zika during pregnancy. Following reports in Brazil of an increase in the number of babies born with microcephaly (small head size) and other serious brain anomalies to mothers infected with Zika, researchers examined the virus as a possible cause. In 2016, the CDC confirmed that, on the basis of the available evidence, Zika can cause birth defects including microcephaly, impaired growth, eye defects, hearing loss, and possibly other problems. The specific geographic areas where Zika virus is spreading are likely to change over time, and research is ongoing into its effects and the best strategies for testing and prevention. The mosquitoes that carry Zika are found in many parts of the world, including much of the United States, and not only are they aggressive daytime biters but they are also active at night. The following recommendations can help you to avoid transmission of the virus: Wear long-sleeved shirts and long pants. Stay in places with air conditioning and window and door screens to keep mosquitoes outside; if screened rooms are not available or if sleeping outdoors, use a mosquito bed net. Treat your clothing and gear with insect repellent, and use approved insect repellents on any exposed skin. Prevent sexual transmission of Zika by using condoms or refraining from sex; researchers have not yet determined how long Zika virus remains in semen after infection. For up-to-date information, visit the CDC Zika website (www.cdc.gov/zika). Treating someone with the highly infectious Ebola virus involves a careful and meticulous process of putting on and taking off protective gear. The use of protective gear is one strategy for breaking the chain of infection. © Bloomberg/Waldo Swiegers/Getty Images Ebola Ebola virus disease (EVD) is caused by Ebola virus, which is transmitted to people from wild animals. Infected people can then transmit the virus to others through direct contact with blood or body fluids or objects that have been contaminated. EVD is a severe infection with an average fatality rate of about 50%. Ebola is rare, but outbreaks have occurred periodically since it was first identified in 1976. There is no vaccine; it is treated with supportive care, and spread of the infection is controlled by identifying and isolating the sick and their close contacts. As of June 2016, about 28,600 cases had been reported worldwide, with about 12,000 deaths. The outbreak in West Africa Page 495that began in March 2014 was the largest in history. The most severely affected countries were Guinea, Liberia, and Sierra Leone. Four cases were treated in the United States: one a traveler from West Africa (the only death in the United States), two health care workers who treated him, and a medical aid worker who had worked in Guinea. On January 14, 2016, the WHO declared that the outbreak was at its end, although occasional flare-ups may still occur. West Nile Virus West Nile virus is carried by birds and then passed to humans when mosquitoes bite first an infected bird and then a person. In the United States, West Nile virus has caused infections in humans in 46 states and the District of Columbia. About 20% of infected people develop fever and other moderate symptoms, such as muscle aches and nausea; less than 1% develop a serious neurological illness such as meningitis or encephalitis. Since 1999, when West Nile virus disease was first identified in the United States, more than 40,000 cases and 1,700 deaths have been reported. PATHOGENIC ESCHERICIA COLI E. coli bacteria live in the intestines of humans and animals and are an essential component of gut immunity. However, some strains of E. coli cause diarrhea; six types cause disease. Since 2006, there have been 24 multistate outbreaks of E. coli-caused disease. These outbreaks have been caused by contaminated foods including lettuce, spinach, sprouts, hazelnuts, processed meats, frozen foods, poultry, and beef sold both in grocery store chains and in restaurants. However, people also can become infected in other ways, such as by swallowing contaminated swimming pool water and through contact with an infected animal—for example, by petting an animal and then not washing hands before eating. Pets and animals at exhibits such as petting zoos can be healthy but carry pathogens on their bodies that can be passed to humans. The CDC website has guidelines for preventing infection. Hantavirus Hantavirus infection can cause a deadly disease called hantavirus pulmonary syndrome (HPS). In North America, the deer mouse, the white-footed mouse, the rice rat, and the cotton rat are carriers of hantaviruses. People can become infected when rodent urine and droppings that contain the virus get into the air, for example, after they dry out. Since 1993, 606 cases of HPS have been counted in the lower 48 states, with an average fatality rate of 36%. Factors Contributing to Emerging Infections What's behind this rising tide of infectious diseases? Contributing factors are complex and interrelated. Drug Resistance New or increasing drug resistance has been found in organisms that cause malaria, tuberculosis, gonorrhea, influenza, AIDS, and pneumococcal and staphylococcal infections. Infections caused by drug-resistant organisms prolong illness, and—if not treated in time—they can cause death. Some bacterial strains now appear to be resistant to all available antibiotics. Poverty More than 1 billion people live in extreme poverty, and half the world's population has no regular access to essential drugs. Population growth, urbanization, overcrowding, and migration (including the movement of refugees) also spread infectious diseases. Breakdown of Public Health Measures A poor public health infrastructure is often associated with poverty and social upheaval, but problems such as contaminated water supplies can occur even in industrial countries. Inadequate vaccination has led to the reemergence of diseases such as diphtheria and pertussis. Natural disasters such as hurricanes also disrupt the public health infrastructure, leaving survivors with contaminated water and food supplies and no shelter from disease-carrying insects. Travel and Commerce International tourism and trade open the world to infectious agents. For example, the ongoing outbreak of Zika virus in Brazil is believed to be due to a combination of increased numbers of foreign travelers attending the 2014 FIFA World Cup and a large population of Aedes mosquitoes in the region. Mass Food Production and Distribution Food now travels long distances to our table, and microbes are transmitted along with it. Mass production of food increases the likelihood that a chance contamination can lead to mass illness. Human Behaviors The widespread use of injectable drugs rapidly transmits HIV infection and hepatitis. Changes in sexual behavior over the past 40 years have led to a proliferation of old and new sexually transmitted infections. The use of day care facilities for children has led to increases in the incidence of several infections that cause diarrhea. Climate Change The lengthening of warm periods and a change to shorter or milder winters may enable pathogenic species and, for some pathogens, their insect vectors to expand their range into countries or states where they once had not existed or had been rare. Immune Disorders Considering the complexity of the immune system, it is not surprising that the system sometimes fails to operate properly, resulting in disease. Autoimmune Diseases As discussed earlier, the immune system can recognize the body's cells and tissues as "self" and foreign organisms as "nonself." When this ability breaks down, "self" may be misread and attacked as "nonself," and the result is an autoimmune disease. In most autoimmune diseases, the immune system targets or destroys specific tissues. For example, in type 1 diabetes, the insulin-producing cells of the pancreas are destroyed. In multiple sclerosis, the protective coating around nerves is destroyed. In Hashimoto's thyroiditis, the thyroid gland is destroyed. In rheumatoid arthritis,Page 496 the membranes lining the joints are destroyed. In systemic lupus erythematosus, destructive inflammation affects the joints, blood vessels, heart, lungs, brain, and kidneys. The causes of autoimmune diseases are not well understood. The rates of many autoimmune diseases are much higher in women than in men. About 2 million Americans have lupus, and 80% of them are women. Among the estimated 1% of Americans with rheumatoid arthritis, women outnumber men three to one. A number of studies in animal models and humans indicate that gut microbes have a role in susceptibility or resistance to the development of autoimmune disease. Because diet can affect the composition of gut microbiota, there is increasing interest in the role of diet in the occurrence of specific autoimmune diseases. The Immune System and Cancer As explained in Chapter 16, cancer cells are cells that have mutated in ways that allow them to multiply uncontrollably. The immune system detects cells that have recently become abnormal and destroys them just as it would a foreign cell. However, some types of cancer cells actually suppress immune responses. In recent years, conventional cancer treatments have been augmented with a treatment known as immunotherapy. The goal of immunotherapy is to stimulate the patient's immune system to attack tumor cells. One immunotherapeutic strategy involves administering tumor-specific antibodies to the patient to stimulate the immune system to recruit macrophages to destroy the tumor cells. Another strategy is to infuse the patient with immune cells, such as dendritic cells. This is the basis of the vaccine for prostate cancer, the first FDA-approved cancer vaccine. This vaccine does not cure cancer but can extend patients' lives by several months. Because some strains of HPV are linked to cervical, anal, and some throat cancers, the commercially available Gardasil and Cervarix vaccines can prevent cancer by stimulating immunity to HPV.

supporting your immune system

SUPPORTING YOUR IMMUNE SYSTEM The immune system does an amazing job of protecting you from illness, but you can help ensure its optimal functioning by choosing healthy behaviors. Here are some general guidelines for supporting your immune system: Eat a balanced diet and maintain a healthy weight, as discussed in Chapter 12 and 14. Get enough sleep. Experimental studies have shown that people who sleep fewer than six hours per night have compromised immune function. For example, people who operate on insufficient sleep are more susceptible to the common cold; they also do not respond as well to immunizations. Quality sleep is as important as sufficient sleep. Sleep quality refers to a collection of measurable components, including the length of time it takes to fall asleep, duration of sleep, the amount of time spent asleep as a proportion of the total time spent in bed, frequency of awakenings, and feeling refreshed or tired upon awakening. Difficulty falling asleep and disrupted sleep can be influenced by environmental factors such as noise, light, and temperature. See Chapter 2 for more on sleep. Exercise, but not when you're sick. Exercise helps you stay healthy, as discussed in Chapter 13. It also staves off stress, which can weaken your immune system. Don't smoke. Smoking decreases the levels of some immune cells (see Chapter 11). If you drink alcohol, do so only in moderation (see Chapter 10). Excessive drinking can interfere with normal immune system functioning. Make sure you get enough Vitamin D. Vitamin D is made in the skin upon exposure to sunlight. It is not always possible to get sufficient sunlight exposure. Wash your hands frequently, as advised throughout this chapter. Antibacterial soap has not been proven to reduce the risk of infection any better than regular soap, especially if you wash properly. When soap and water are not available, use hand sanitizer. Make sure the product is at least 60% alcohol, but avoid sanitizers that contain triclosan. Triclosan has been detected in the breast milk, urine, and plasma of people who use products containing it. This presence is worrisome because it has been linked to thyroid dysfunction. Triclosan may also affect the immune system, because it can exacerbate asthma in response to environmental allergens. Avoid contact with people who are contagious with an infectious disease. Make sure you drink water only from clean sources. Unpurified water from lakes and streams can carry pathogens, even if it seems pristine. Avoid contact with disease carriers such as rodents, mosquitoes, and ticks. Never touch or feed wild animals or rodents. Practice safer sex (as described in Chapter 18). Do not use injectable drugs of any kind (see Chapter 9). Make sure you have received all your recommended vaccinations, and keep them up-to-date. Your physician can tell you exactly what immunizations you need and when you should have them. Ask Yourself QUESTIONS FOR CRITICAL THINKING AND REFLECTION Have you ever had any of the illnesses described in the preceding sections? How were you exposed to the disease? Could you have taken any precautions to avoid it?

The Bodys defense system

THE BODY'S DEFENSE SYSTEM Our bodies have very effective ways of protecting themselves against invasion by foreign organisms. The immune system is the body's collective set of defenses that includes surface barriers as well as the specialized cells, tissues, and organs that carry out the immune response. The first line of defense is a formidable array of physical and chemical barriers. When these barriers are breached, cellular processes of the immune system come into play. Together these defenses provide an effective response to nearly all the invasions that our bodies experience. Physical and Chemical Barriers The skin, the body's largest organ, prevents many microorganisms from entering the body. Although many bacterial and fungal organisms live on the surface of the skin, few can penetrate it except through a cut or break. All body cavities and passages that are exposed to the external environment are lined with mucous membranes, which Page 473secrete mucus and contain cells designed to prevent the passage of unwanted organisms and particles. These areas include the mouth, nostrils, eyelids, bronchioles, vagina, and other organs of the respiratory, digestive, and urogenital tracts. Skin and mucous membranes are made of epithelial tissue, which consists of one or more layers of closely packed cells with almost no space between cells. The fluids that cover epithelial tissue, such as tears, saliva, and vaginal secretions, are rich in enzymes and other proteins that break down and destroy many microorganisms. The respiratory tract is lined not only with mucous membranes but also with cells having hairlike protrusions called cilia. The cilia sweep foreign matter up and out of the respiratory tract. Particles that are not caught by this mechanism may be expelled from the system by a cough. The Immune System: Cells, Tissues, and Organs Beyond surface barriers, the immune system operates through a remarkable information network involving billions of white blood cells that protect the body when a threat arises. These white blood cells are produced continuously throughout life. The immune system can be thought of as two systems. The innate immune system consists of cells that can recognize pathogenic microorganisms and are the first responders to those pathogens. The adaptive immune system consists of cells (called T cells and B cells) that can recognize pathogenic microorganisms and that have the remarkable ability to improve and accelerate their responses after exposure to those pathogens. The complete elimination of a pathogen involves the coordinated activities of both systems. An important cell type that is at the nexus of the innate and adaptive immune systems is the dendritic cell. Cells of the Innate Immune System The cells of the innate immune system recognize pathogens as "foreign" and kill them, but they cannot develop a memory of these pathogens. Thus, they respond the same way no matter how many times a pathogen invades. The innate immune system includes the following types of cells: Neutrophils, one type of white blood cell, travel in the bloodstream to areas of invasion, attacking and ingesting pathogens. Eosinophils, white blood cells that occur in mucosal tissues such as those in the gastrointestinal tract and the mammary glands, provide innate immunity to certain microbes. Macrophages, or "big eaters," act as scavengers, devouring pathogens and worn-out cells. Natural killer cells directly destroy virus-infected cells and cells that have turned cancerous. Dendritic cells, which reside in tissues, engulf pathogens and activate lymphocytes. Cells of the Adaptive Immune System The cells of the adaptive immune system are white blood cells called lymphocytes. The two main types of lymphocytes are T cells and B cells, which differ in function. Whereas cells of the innate immune system can recognize a cell as foreign, lymphocytes are capable of exquisite specificity and of immunological memory. Antigens All of your body cells display markers on their surfaces that identify them as "self" to lymphocytes. Invading microbes also display markers, and these markers identify them as foreign, or "nonself," to lymphocytes. Nonself markers that trigger an immune response are known as antigens. Antibodies Antibodies are specialized proteins that circulate in your bloodstream and are present in most body fluids. Produced by B cells, antibodies have complementary markers on their surface that bind to antigens on microbes. Antibodies do not destroy microbes directly, but by binding to antigens, antibodies tag the microbe for destruction by other types of immune cells. T Cells and B Cells Each T and B cell has receptors that allow it to recognize one specific antigen. The immune system Page 474produces T and B cells with many receptor types, each capable of recognizing a different antigen, meaning the immune system can recognize nearly all disease-causing microbes. When a B cell lymphocyte or T cell lymphocyte encounters the antigen for which it is specific, it proliferates, producing many more lymphocytes that are specific to the same antigen. These daughter cells then differentiate into cells with specific immune functions and attack the invading organisms. B cells become plasma cells that secrete antibodies. T cells differentiate into helper T cells, killer T cells, or suppressor T cells (also called regulatory T cells). Some B and T cells become memory B and T cells, which can mount a rapid and powerful response should they encounter the same invader months or even years in the future. A dendritic cell is a white blood cell that engulfs foreign cells and displays their antigens in a way that makes these antigens recognizable by B and T cells. This process activates the B and T cells, signaling them to proliferate and respond to the specific invader. The dendritic cell shown in this scanning electron micrograph is magnified 2700 times. © Eye of Science/Science Source The Inflammatory Response When injured or infected, the body reacts by producing an inflammatory response. Macrophages engulf the invading microbe and produce substances that convey danger to other cells of the immune system. The resulting inflammatory response triggered by histamine (a chemical that also contributes to the allergic response) causes blood vessels to dilate and fluid to flow out of capillaries into the injured tissue. This activity produces increased heat, swelling, and redness in the affected area. White blood cells are drawn to the area and attack the invaders, in many cases destroying them. This entire reaction is the immune response, as described in the next section. At the site of infection there may be pus—a collection of dead white blood cells and debris resulting from the encounter. The Immune Response The activities of the innate and adaptive immune systems are integrated to generate a coordinated and usually highly effective immune response. Figure 17.1 illustrates how this happens: FIGURE 17.1 The immune response. Once invaded by a pathogen, the body mounts a complex series of reactions to destroy the invader. Pictured here are the phases of the immune response as the body works to destroy a virus. Page 475Phase 1: Recognition. If a pathogen breaches the body's physical and chemical barriers, it initiates the first phase of the immune response by arousing dendritic cells at the site of pathogen entry. They engulf the pathogen and migrate to nearby lymphoid tissue. There the dendritic cells activate helper and killer T cells by presenting fragments of pathogen proteins to T cells. Antigen carried into lymphoid tissue also activates B cells. Phase 2: Proliferation. The activated helper and killer T cells multiply, thereby amplifying the immune response to the pathogen. Helper T cells produce special growth stimulants called cytokines, which then further stimulate the activation and proliferation of killer T cells and B cells. Phase 3: Elimination. The activated T and B cells then undergo a transformation to become either memory cells or effector cells. The effector cells mediate elimination of the pathogen. If the infecting pathogen is a virus or an intracellular bacterium, then killer T cells destroy body cells that are infected with that pathogen. Activated B cells become memory B cells or plasma cells, which are antibody-producing factories. The antibodies bind to extracellular pathogens (those outside body cells) and mark them for destruction by macrophages and natural killer cells. Phase 4: Slowdown. Regulatory T cells inhibit lymphocyte proliferation and induce lymphocyte death, causing a slowdown of the immune response. This process restores "resting" levels of B and T cells. Some memory T and B cells remain and can initiate a rapid response if the same pathogen reappears in the future. Immunity Usually, after an infection, a person is immune to the same pathogen. This immunity, or insusceptibility, occurs because some of the lymphocytes created during phase 2 of the immune response are reserved as memory T and B cells. They continue to circulate in the blood and lymphatic system for years or even for the rest of the person's life. If the same antigen enters the body again, the memory T and B cells recognize and destroy it before it can cause illness. The ability of memory lymphocytes to remember previous infections and improve immune defenses if the same microbe is encountered in the future is known as adaptive immunity. The Lymphatic System The lymphatic system consists of a network of vessels that carry a clear fluid called lymph. It also includes organs and structures, such as the spleen and the lymph nodes, that function as part of the immune system (Figure 17.2). The lymphatic vessels pick up excess fluid from body tissues; this fluid may contain microbes and dead or damaged body cells. Macrophages, dendritic cells, and lymphocytes congregate in the lymph nodes; when lymph passes through a node, foreign cells and debris are filtered out and destroyed. If immune cells in a lymph node recognize an antigen, the adaptive immune response will be triggered. As the immune response progresses, a lymph node actively involved in fighting an infection may fill with cells and swell. Physicians use the location of swollen lymph nodes as a clue to an infection's location. Image details[D] FIGURE 17.2 The lymphatic system. The lymphatic organs are production centers for infection-fighting cells and sites for some immune responses. Page 476 Immunization The ability of the immune system to remember previously encountered organisms and retain its strength against them is the basis for immunization. When a person is immunized, the immune system is primed with an antigen similar to the pathogenic organism. The body responds by producing antibodies, which prevent serious infection if the person is exposed to the disease organism itself. The preparations used to manipulate the immune system are known as vaccines. Table 17.1 summarizes the vaccines currently recommended for most adults. Visit the Centers for Disease Control and Prevention (CDC) Vaccines & Immunizations website (www.cdc.gov/vaccines) for updates and the recommendations for other groups, including children, travelers, pregnant women, and adults with special health risks. Some vaccines require multiple doses or periodic booster shots to maintain effectiveness, so it is important to keep up with the recommended vaccine schedule. Table 17.1 Immunizations Recommended for Adults (Age 19 and Over)* VACCINE RECOMMENDATION Flu All adults, every year Td/Tdap (tetanus, diphtheria, pertussis) All adults, Td booster every 10 years, substitute Tdap for Td once HPV (human papillomavirus) Women up to age 26, men up to age 21, and men aged 22-26 who have sex with men who have not already completed the vaccine series Shingles Adults age 60+, including those who have had shingles Pneumococcal Adults age 65+ Meningococcal Adults at risk, including unvaccinated college students living in dormitories MMR (measles, mumps, rubella) Adults born after 1957 who were not vaccinated as children and who lack evidence of past infection Chickenpox Adults not vaccinated as children who lack evidence of past infection Hepatitis A Adults with specific health conditions or risk factors who were not vaccinated as children Hepatitis B Adults with specific health conditions or risk factors who were not vaccinated as children *For international travelers, all standard childhood immunizations should be up-to-date and additional vaccines considered. Information for travelers and adults with special health concerns is available from the CDC's national immunization program (800-232-2522; www.CDC.gov/vaccines/) and the CDC's travelers' health resources (877-394-8747; www.cdc.gov/travel). The CDC has interactive tools (at www.cdc.gov/vaccines) to help individuals decide which vaccines they need. source: Centers for Disease Control and Prevention. 2016. Immunization Schedules 2016 (http://www.cdc.gov/vaccines/schedules/index.html). Immunization has become a source of public debate among people who question whether vaccinations do more harm than good. They worry in particular about the efficacy and safety of certain types of vaccinations. Public health officials are concerned because decreases in vaccination rates can result in outbreaks of dangerous infectious diseases, especially since international travel makes it easy for pathogens to cross borders from countries with low vaccination rates. Large outbreaks have occurred in other countries in recent years. For example, France had an outbreak of measles in 2008-2011 (more than 20,000 cases; 5000 hospitalizations; and 10 deaths), and U.S. travelers to France brought measles home with them. Vaccine-preventable diseases still exist around the world, and if vaccination rates drop, diseases can become much more common. Types of Vaccines Vaccines can be made in several ways. In some cases, microbes are cultured in the laboratory in a way that attenuates (weakens) them. These live, attenuated organisms are used in vaccines against diseases such as measles, mumps, and rubella (German measles). In other cases, when it is not possible to generate attenuated organisms, vaccines are made from pathogens that have been killed but that still retain their ability to stimulate the production of antibodies. Vaccines composed of killed viruses are used against influenza viruses, among others. Active immunity, a condition in which the vaccinated person produces his or her own antibodies to the microorganism, can be acquired by vaccination or naturally when a person is infected and mounts an immune response to the pathogen. This contrasts with passive immunity, which is conferred by injecting gamma globulin (antibodies) produced by other human beings or animals who have recovered from a given disease. Passive immunity does not confer memory and provides only short-term protection against a pathogen. Vaccine Efficacy In the past century, vaccines have helped to increase the average American lifespan by approximately 30 years—and they have done so with an excellent safety record. Nevertheless, misinformation about vaccines has spread (particularly on the Internet and in popular media), raising concerns about their efficacy and safety. The role of vaccines in human health is underappreciated, and their success in preventing disease has been well established. Most childhood vaccines are effective enough to prevent disease in most people. For a small proportion of people, a vaccine is not effective—that is, the vaccine does not provoke a strong enough immune response. For example, Page 477for measles, an estimated 3 out of 100 vaccinated people are not fully protected, although if infected, they are likely to have milder illness and are less likely to spread the disease compared to someone who is not vaccinated. Protection from infection is provided by those around them, as long as they live, work, and travel around people who have been vaccinated and are themselves protected. The same is true for people who cannot be vaccinated due to young age or underlying medical conditions (such as cancer treatment). Keeping vaccination rates consistently high over time is necessary to maintain protection. Before the development of the measles vaccine in 1963, nearly everyone in the United States got measles, and hundreds died from measles every year. In the five years before the vaccine was introduced, about 400-500 deaths and 48,000 hospitalizations from measles occurred annually. Following widespread vaccination, the United States was declared measles-free in 2000, meaning measles is not constantly present. However, outbreaks have continued to occur, with infection brought into the United States by unvaccinated travelers (Americans or foreign visitors) who get measles while they are in other countries. People with measles can easily spread it to others who are not vaccinated or otherwise protected (e.g., from past infection). In an outbreak in 2011, over 70% of cases were traced to travel abroad by U.S. citizens, nearly half of whom acquired measles in Europe. A more recent outbreak of more than 100 cases in 2014-2015 originated in Disney theme parks in California; the majority of the people affected were unvaccinated or missing doses, either by choice or because they were too young. About 20% of patients in the outbreak were hospitalized due to serious illness. QUICK STATS Within 6 years of the HPV vaccine's introduction, the prevalence of HPV infection in vaccinated sexually active 14- to 24-year-old females in the United States dropped from 19% to 2%. —Markowitz, et al., Pediatrics, 2016 Vaccine Safety Vaccines are approved by the Advisory Committee on Immunization Practices—the advisors for the CDC—and the Committee on Infectious Diseases—the advisors for the American Academy of Pediatrics (AAP). Both the AAP and the CDC advisory committees have expert knowledge in virology, microbiology, statistics, epidemiology, and pathogenesis—knowledge necessary for reviewing and evaluating studies on vaccine efficacy and safety. To further ensure their safety, vaccines are tested in larger numbers of people for longer periods of time than drugs are before being licensed. For example, the human papillomavirus (HPV) vaccine was tested in 30,000 women, the conjugate pneumococcal vaccine was tested in 40,000 children, and each of the current rotavirus vaccines was tested in 70,000 children before being licensed. No other type of medication receives this degree of scrutiny. Additionally, safety mechanisms such as the Vaccine Adverse Event Reporting System (VAERS) and the Vaccine Safety Datalink Project monitor adverse events reported after licensure. Side effects from immunization are usually mild, such as soreness at the injection site. It is estimated that an allergic reaction may occur in 1 in 1.5 million doses. Any risk from a vaccine must be balanced against the risk posed by the diseases it prevents. For example, the last major U.S. epidemic of rubella in 1964-1965 infected 12.5 million Americans, caused 11,000 miscarriages, killed 2000 infants, and resulted in 20,000 infants being born with rubella-caused birth defects. Since the introduction of the vaccine and the maintenance of high vaccination rates, U.S. rubella cases have dropped to an average of about 10 per year. Worldwide, however, an estimated 100,000 babies are born each year with birth defects due to rubella infection in pregnant women. For more on the safety, efficacy, and testing of vaccines, visit the CDC Vaccines & Immunizations website (www.cdc.gov/vaccines) and the website for the American Academy of Pediatrics (www.aap.org). Allergy: A Case of Mistaken Identity An estimated 50 million Americans are affected by allergies. In a person with an allergy, the immune system reacts to a harmless substance as if it were a harmful pathogen. For the most part, allergy symptoms—stuffy nose, sneezing, wheezing, skin rashes, and so on—are the result of the immune response. Allergens Substances that provoke allergies are known as allergens; they may cause a response if they are inhaled or swallowed or if they come in contact with the skin. Page 478More than half of Americans aged 6-59 react to at least one common allergen. Common allergens include the following: Pollen. Referred to as hay fever or allergic rhinitis, pollen allergies are widespread. Weeds, grasses, and trees are common producers of allergenic pollen. Animal dander. People with animal allergies are usually allergic not to fur but to dander (dead skin flakes), urine, or a protein found in saliva. Allergies to mice, dogs, and cats are common. Dust mites and cockroaches. The droppings of cockroaches and microscopic dust mites can trigger allergies. Mites live in carpets, upholstered furniture, and bedding. Molds and mildew. The small spores produced by these fungi can trigger allergy symptoms. Molds and mildew thrive in damp areas of buildings. Foods. The most common food allergens in adults include peanuts, tree nuts, fish, and shellfish. Insect stings. The venom of insects such as yellow jackets, honeybees, hornets, paper wasps, and fire ants causes allergic reactions in some people. People may also be allergic to certain medications, plants such as poison oak, latex, metals such as nickel, and compounds found in cosmetics. The Allergic Response Although allergies are an adverse response to environmental antigens that are not pathogenic, this type of immune response in fact evolved as a protective response to certain types of pathogens, particularly parasitic worms. Most allergic reactions are due to the production of a special type of antibody known as immunoglobulin E (IgE). Initial exposure to a particular allergen sensitizes the immune system by causing the production of allergen-specific IgE, which binds to special IgE receptors on mast cells (Figure 17.3). When the body is subsequently exposed to the allergen, the allergen binds to IgE antibodies that are bound by IgE receptors on certain white blood cells called mast cells, thereby causing the mast cells to release large amounts of histamine and other compounds that cause allergic symptoms. FIGURE 17.3 The allergic response. QUICK STATS About 8.6% of children and 7.4% of adults currently have asthma in the United States. —Centers for Disease Control and Prevention, 2016f Histamine has many effects, including increasing the inflammatory response and stimulating mucus production. The precise symptoms of an allergic reaction depend on what part of the body is affected. In the nose, histamine may cause congestion and sneezing; in the eyes, itchiness and tearing; in the skin, redness, swelling, and itching; in the intestines, bloating and cramping; and in the lungs, coughing, wheezing, and shortness of breath. In some people, an allergen can trigger an asthma attack (see the box "Poverty, Ethnicity, and Asthma"). Symptoms—wheezing, tightness in the chest, shortness of breath, and coughing—often occur immediately, within minutes of exposure, but inflammatory reactions may take hours or days to develop and then may persist for several days. The symptoms may be mild and occur only occasionally, or they may be severe and occur daily. DIVERSITY MATTERS: Poverty, Ethnicity, and Asthma In 2014, asthma affected 24 million people in the United States (17.7 million adults and 6.3 million children under age 18), or 7.7% of the population. Since 1980, the number of Americans with asthma has been increasing steadily, despite better treatments and cleaner air. Prevalence The prevalence of asthma is higher among children, women, African Americans, Puerto Ricans, and people whose family income is below the poverty line. African Americans are four times more likely to be hospitalized with asthma than are other ethnic groups and five times more likely to die from it. People with low incomes are especially at risk; they are more often exposed to the environmental factors that contribute to asthma, such as air pollutants, allergens indoors, and air pollution from highways, incinerators, and waste facilities outdoors. Low-income groups are also less likely to have access to health care. Treatment and Prevention of Attacks The symptoms of an asthma attack can be relieved by the use of a bronchodilator—a fast-acting medication, delivered through an inhaler, that opens the airways. Inhaled corticosteroids and other prescribed medications are needed to treat the underlying inflammation of the airways. People with asthma can monitor their condition by self-testing their peak air flow, the speed at which they can exhale, using a special device. A drop in peak air flow can signal an upcoming attack. © moodboard/Corbis RF Some simple measures can help people avoid allergens and triggers. These measures include using HEPA (high-efficiency particulate air) filters in vacuum cleaners and room air purifiers, covering mattresses and pillows with special covers, avoiding tobacco smoke, and using professional pest control. sources: Centers for Disease Control and Prevention. 2016. Most Recent Asthma Data: National Data (http://www.cdc.gov/asthma/most_recent_data.htm); Akinbami, L. J., A. E. Simon, and K. C. Schoendorf. 2015. Trends in allergy prevalence among children aged 0-17 years by asthma status, United States, 2001-2013. Journal of Asthma 15: 1-21. Asthma is caused by both chronic inflammation of the airways and spasm of the muscles surrounding the airways. The spasm causes constriction, and the inflammation causes the airway linings to swell and secrete extra mucus, which further obstructs the passages. An asthma attack is initiated by an irritating stimulus in the bronchial tubes. The stimulus may be an inhaled allergen, such as pollen, dust mites, mold, animal dander, or cockroach droppings, or it can be a Page 479nonallergen stimulus, such as exercise, cold air, pollutants, tobacco smoke, infection, or stress. In women with asthma, hormonal changes that occur as menstruation starts may increase vulnerability to attacks. Both environmental and genetic factors contribute to the development of asthma. The most serious, but rare, kind of allergic reaction is anaphylaxis, which results from a release of histamine throughout the body. Anaphylactic reactions can be life threatening because symptoms may include swelling of the throat, extremely low blood pressure, fainting, heart arrhythmia, and seizures. Anaphylaxis is a medical emergency, and treatment requires immediate injection of epinephrine. People at risk for anaphylaxis should wear medical alert identification and keep self-administrable epinephrine readily available. Climate Change and Allergies The predicted global changes in climate are likely to exacerbate allergies, including allergic rhinitis and asthma. Global warming is influencing plant and fungal reproduction and thus is influencing both spatial and temporal production of allergens. If warm weather periods lengthen, then the period of pollen release also will be prolonged and the amount of pollen released may increase. Thus, politicians and policymakers must become involved in reducing the impact of climate change on human health. As a first step, in 2006 the World Health Organization (WHO), governments of some countries, scientific organizations, and patient advocacy groups formed the Global Alliance against Chronic Respiratory Disease (GARD). One goal of GARD is to prevent or at least lessen risk factors (such as tobacco smoke and other pollutants) that exacerbate allergic responses to allergens. Page 480 Dealing with Allergies If you suspect you might have an allergy, visit your physician or an allergy specialist. There are three general strategies for dealing with allergies: Avoidance. You may be able to avoid or minimize exposure to allergens by changing your environment or behavior. For example, removing carpets from the bedroom and using special bedding can reduce dust mite contact. Pollen exposure can be limited by avoiding outdoor activities during peak pollination times, keeping windows shut, and showering and changing clothes following outdoor activities. If you can't part with a pet, keep pets out of bedrooms and frequently vacuum or damp-mop floors. Medication. A variety of medications are available for allergy sufferers. Many over-the-counter antihistamines are effective at controlling symptoms such as blocked nasal, sinus, or middle ear passages. Prescription corticosteroids delivered by aerosol markedly reduce allergy symptoms, increase effectiveness, and help limit systemic absorption and side effects. Immunotherapy. Referred to as "allergy shots," immunotherapy desensitizes a person to a particular allergen through the administration of gradually increasing doses of the allergen over a period of months or years. Ask Yourself QUESTIONS FOR CRITICAL THINKING AND REFLECTION What are your views on government-required child vaccinations? What has shaped your views on this issue?

the spread of diease

THE SPREAD OF DISEASE The immune system is operating at all times, maintaining its vigilance when you're well and fighting invaders when you're sick. How does it all feel to you, the host? And how do diseases spread from one person to another, sometimes resulting in epidemics or pandemics? These questions are addressed in this section. Symptoms and Contagion The symptoms you experience during an illness are related to the phase of infection and the actions of your immune system. During the first phase of infection, or incubation period, when viruses or bacteria are actively multiplying before the immune system has gathered momentum, you may not have any symptoms of the illness, but you may be contagious. During the second and third phases of the immune response, you may still be unaware of the infection, or you may "feel a cold coming on." Symptoms first appear during the prodromal period, which follows incubation. If you have acquired immunity, the infection may be eradicated during the incubation period or the prodromal period. In this case it does not develop into a full-blown illness. Many symptoms of an illness are actually due to the body's immune response rather than to the actions or products of the invading organism. For example, fever is caused by the release of certain cytokines by macrophages and other cells during the immune response. These cytokines travel in the bloodstream to the brain and cause the body's thermostat to be reset to a higher level. The resulting elevated temperature helps the body fight against pathogens by enhancing immune responses. (During an illness, it is necessary to lower a fever only if it is uncomfortably high [over 101.5°F] or if it occurs in an infant who is at risk for seizures from fever.) Similarly, you get a runny nose when your lymphocytes destroy infected mucosal cells, leading to increased mucus production. The malaise and fatigue of the flu are caused by pro-inflammatory cytokines. You may be contagious before you experience any symptoms, and you are contagious as long as your body is releasing infectious microbes. This means that you can transmit an illness without knowing you're infected or catch an illness from someone who doesn't appear to be sick. Similarly, your symptoms may continue after the pathogens have been mostly destroyed, when you are no longer infectious. The Chain of Infection Infectious diseases are transmitted from one person to another through a series of steps—a chain of infection (Figure 17.4). New infections can be prevented by interfering with any step in this process. Figure displays the links in the chain of infection along with strategies for breaking each link in the chain.[D] FIGURE 17.4 The chain of infection. Any break in the chain of infection can prevent disease. Links in the Chain The chain of infection has six major links: Pathogen. The infectious disease cycle begins with a pathogen that enters the body. Many pathogens cause illness by invading body cells; others cause illness by producing toxins that harm tissue. Reservoir. The pathogen has a natural environment—called a reservoir—in which it typically lives. This reservoir can be a person or an animal. A person who is the reservoir for a pathogen may be ill or may be an asymptomatic carrier who, although having no symptoms, can spread infection. Portal of exit. To transmit infection, the pathogen must leave the reservoir through some portal of exit. In the case of a human reservoir, portals of exit include saliva, the mucous membranes, blood, feces, and nose and throat discharges. Means of transmission. Transmission can occur directly or indirectly. In direct transmission, the pathogen is passed from one person to another without an intermediary. Most common respiratory infections and many intestinal infections are passed directly—for example, when a person with an infectious agent on his or Page 481her hands touches someone else. Other means of direct transmission include sexual contact and contact with blood. In indirect transmission, animals or insects such as rats, ticks, and mosquitoes serve as vectors, carrying the pathogen from one host to another. Pathogens can also be transmitted via contaminated soil, food, or water or from inanimate objects, such as eating utensils, doorknobs, and handkerchiefs. Some pathogens float in the air for long periods, suspended on tiny particles of dust or droplets that can travel long distances before they are inhaled and cause infection. Portal of entry. To infect a new host, a pathogen must have a portal of entry into the body. Pathogens can enter through direct contact with or penetration of the skin or mucous membranes, inhalation, or ingestion. Pathogens that enter the skin or mucous membranes can cause a local infection of the tissue, or they may penetrate into the bloodstream or lymphatic system, thereby causing a systemic infection. Agents that cause sexually transmitted infections usually enter the body through the mucous membranes lining the urethra (in males) or the cervix (in females). Organisms that are transmitted via respiratory secretions may cause upper respiratory infections or pneumonia, or they may enter the bloodstream and cause systemic infection. Foodborne and waterborne organisms enter the mouth and travel to the tissue that will best support their reproduction. They may attack the cells of the small intestine or the colon, causing diarrhea, or they may enter the bloodstream via the digestive system and travel to other parts of the body. The new host. Once in the new host, a variety of factors determine whether the pathogen will be able to establish itself and cause infection. People with a strong immune system or resistance to a particular pathogen are less likely to become ill than are people with poor immunity. If conditions are right, the pathogen will multiply and produce disease in the new host. In such a case, the new host may become a reservoir from which a new chain of infection can be started. Breaking the Chain Interrupting the chain of infection at any point can prevent disease. Strategies for breaking the chain include both public health measures and individual action. For example, a pathogen's reservoir can be isolated or destroyed, as when a sick individual is placed under quarantine or when insects or animals carrying pathogens are killed. Public sanitation practices, such as sewage treatment and the chlorination of drinking water, can also kill pathogens. Transmission can be disrupted through strategies like hand washing and the use of face masks. Immunization and the treatment of infected hosts can stop the pathogen from multiplying, producing a serious disease, and being passed on to a new host. Figure 17.4 lists methods of breaking the chain of infection. Epidemics and Pandemics The rapid spread of a disease or health condition is called an epidemic. Although the word is usually used in reference to infectious diseases, it is also used for health conditions that are not caused Page 482by an infectious organism. For example, it is often said that obesity and diabetes have reached epidemic proportions in the United States. An important underlying premise of the concept of "epidemic" is that the occurrence of the disease is greater than what is expected normally. Thus, the common cold, which occurs with great frequency, is never classified as an epidemic. Conversely, the term epidemic is used to refer to outbreaks of diseases that are not widespread. For example, when 10 infants died in California in 2010 from whooping cough, the incident was referred to as an epidemic. When an epidemic is widespread, it is called a pandemic. A pandemic is an epidemic that has spread across a large area, such as an entire nation, a continent, or even the world. In contrast to the term epidemic, the term pandemic refers exclusively to infectious disease. Human history has been punctuated by numerous pandemics of various diseases, including bubonic plague, smallpox, and influenza. One of the most severe influenza pandemics occurred in 1918-1919, following World War I, when 20-40% of the world's population became ill and as many as 40 million people died. Not all widespread infectious diseases are pandemics. An infectious disease is said to be endemic when it habitually exists in a certain region. Endemic diseases generally occur at low frequency in particular populations or regions. For example, malaria is endemic to low-altitude areas of northern and eastern South Africa. Seasonal epidemics of malaria can occur in these areas where malaria is endemic. In other words, there are seasonal spikes in the number of malaria cases in these endemic areas. H1N1 Influenza In 2009, an outbreak of influenza caused by a virus never before identified as a cause of human illness sparked fears of a worldwide influenza pandemic. Most people had no immunity to this new strain, initially called swine flu and subsequently labeled H1N1 influenza A virus. The influenza A virus mutates frequently during replication, and it can also exchange genes with other influenza A viruses, including strains that infect domestic and wild animals. The H1N1 strain resulted from a combination of genes from four viruses: two from swine flu viruses, one from an avian (bird) flu virus, and one from a human flu virus. The addition of the gene from a human virus meant that humans could be infected with it, and, because it was new, they had no prior immunity. In June 2009, with cases reported in 74 countries, the World Health Organization declared that a flu pandemic was under way. By February 2010, the WHO reported up to 86 million cases and up to nearly 18,000 deaths. By May of that year, flu activity had tapered off and declined to normal levels, but the same influenza A (H1N1) virus was the predominant strain during the 2013-2014 influenza season. The WHO continues to track trends globally, and the CDC continues to recommend H1N1 vaccination for persons aged 6 months to 24 years and people aged 25-64 years who are at high risk. H5N1 Influenza Since 1998, scientists have been monitoring the progress of another influenza A virus known as H5N1, or avian (bird) influenza. This strain infects chickens, ducks, and geese as well as wild birds; it doesn't pass easily to humans or among humans, but when it does, it is deadly. According to the WHO, more than 700 humans have been infected with Asian H5N1 virus since November 2003. The infections have occurred primarily in Indonesia, Vietnam, and Egypt, although European countries also reported infections. The first case of Asian H5N1 in the Americas was in a traveler who went from China to Canada in January 2014. Although H5N1 infections of humans have been rare, approximately 60% of the infected people died. Ask Yourself QUESTIONS FOR CRITICAL THINKING AND REFLECTION Think about the last time you were sick with a cold, the flu, or an intestinal infection. Can you identify the reservoir from which the infection came? What vector, if any, transmitted the illness to you? Did you pass the infection to anyone else? If so, how?

intro

Countless microscopic organisms live around, on, and in us. Although most microbes are beneficial, many of them can cause infections and infectious diseases. But the constant vigilance of our immune system keeps them at bay and our bodies healthy. The immune system protects us not just from pathogens but also from cancer. In the past decade, the reemergence of old scourges such as tuberculosis and the appearance of new viruses highlight the importance of understanding both the immune system and the pathogens that can cause us harm. This chapter introduces you to the mechanisms of immunity and infection, as well as strategies for keeping yourself well in a world of disease-causing microorganisms.