micro final unit five chapter 5.6 and 5.7, 5.8 and 23 wallace
23.7)Compare the vectors and the pathologies involved in sleeping sickness, Chagas disease, and leishmaniasis
Obligate parasites that live in blood and tissues of human host Cause life-threatening and debilitating zoonoses Spread in specific tropical regions by blood-sucking insects that serve as intermediate hosts Have complicated life cycles and undergo morphological changes Categorized according to cellular and infective stages Members of this group include species of Trypanosoma and Leishmania Hemoflagellates Stages Amastigote: the form lacking a free flagellum Promastigote: the stage bearing a single, anterior flagellum Epimastigote: the flagellate stage Trypomastigote: large, fully formed stage Hemoflagellates Stages TABLE 23.3 Cellular and Infective Stages of the Hemoflagellates Trypanosoma Species and Tropanosomiasis Trypanosoma species are distinguished by their infective stage: Trypomastigote is an elongate, spindle-shaped cell with tapered ends, eel-like motility 2 types of trypanosomiasis: T. brucei - African sleeping sickness T. cruzi - Chagas disease - endemic to Central and South America Trypanosoma brucei & Sleeping Sickness Spread by tsetse flies Two variants of disease caused by 2 subspecies Biting of fly inoculates skin with trypomastigotes, which multiplies in blood and damages spleen, lymph nodes, and brain Harbored by mammals Chronic disease symptoms are sleep disturbances, tremors, paralysis, and coma Diagnosis - Trypanosomes are readily demonstrated in blood, spinal fluid, or lymph nodes Treatment - before neurological involvement melarsoprol, eflornithine Control - involves eliminating tsetse fly Trypanosoma brucei and Sleeping Sickness The distribution of African trypanosomiasis The saliva of a fly infected with T. brucei inoculates the human bloodstream. The parasite matures and invades various organs. In time, its cumulative effects cause central nervous system (CNS) damage. The trypanosome is spread to other hosts through another fly in whose alimentary tract the parasite completes a series of developmental stages. Trypanosoma cruzi & Chagas Disease 5 Reduviid bug (kissing bug) is the vector Central and South America Mexico and southern parts of the US Infection occurs when bug feces are inoculated into a cutaneous portal Local lesion, fever, and swelling of lymph nodes, spleen, and liver Heart muscle and large intestine harbor masses of amastigotes Chronic inflammation occurs in the organs (especially heart and brain) Treatment nifurtimox and benzonidazole Leishmania Species & Leishmaniasis Leishmaniasis - zoonosis transmitted among mammalian hosts by female sand flies that require a blood meal to produce eggs Endemic to equatorial regions Leishmania Species and Leishmaniasis Promastigotes are injected by sand fly bite, then they convert to amastigote and multiply If macrophage does not migrate the infection is localized; systemic if macrophage migrates Leishmania Species and Leishmaniasis Cutaneous-oriental sore, Baghdad boil - localized ulcerated sore Espundia - skin and mucous membrane infection of the head; chronic infection Systemic (visceral) - high intermittent fever; weight loss, enlarged spleen, liver, and lymph nodes Kala azar is the most severe and fatal form if untreated
23.5)Describe the important characteristics of Trichomonas vaginalis and the pathology of this infection.
Small, pear-shaped protozoa 4 anterior flagella and an undulating membrane Exist only in trophozoite form 3 infect humans: T. vaginalis T. tenax T. hominis Trichomonas vaginalis Causes an STD called trichomoniasis Reservoir is human urogenital tract 50% of infected are asymptomatic Strict parasite, cannot survive long outside of host 3 million cases yearly, a top STD Female symptoms - foul-smelling, green-to-yellow discharge; vulvitis; cervicitis; urinary frequency and pain Male symptoms - urethritis, thin, milky discharge, occasionally prostate infection Treatment - Oral or vaginal metronidazole
23.2) Explain the identifying attributes of the four commonly recognized groups of protozoa.
1-Ameoboids - Sarcodina - Pseudopods 2-Flagellates 3-Ciliates 4-Apecomplexians - nonmotile
23.17) Identify risk factors for contracting trichinellosis, and list symptoms of infection
Acquired from eating undercooked pork or bear meat Larvae migrate from intestine to blood vessels, muscle, heart, and brain, where it forms cysts First symptoms - flulike, diarrhea Second symptoms - muscle and joint pain, shortness of breath, pronounced eosinophilia Initial stages treated w/anti-helmenthics No cure after larva have encysted
23.11)Describe the basic characteristics of parasitic helminths. Identify the three categories of parasitic helminthes
Adults are large, multicellular animals with specialized tissues and organs Adult worms mate and produce fertilized eggs that hatch; larvae then mature in several stages to adults The sexes may separate or hermaphroditic Adulthood and mating occur in the definitive host Larval develop occurs in the intermediate host A transport host experiences no parasitic development Four basic patterns of life and transmission Parasitic helminths are grouped into three categories: 1 Nematodes (round worms) 2 Trematodes (flukes) 3 Cestodes (tapeworms)
23.4)Contrast the pathogeneses, treatments, the portals of entry and common outcomes for each pathogenic amoeba (Entamoeba, Naegleria, and Acanthamoeba infections).
Alternates between a large motile trophozoite, and a smaller nonmotile cyst Humans are the primary hosts Cysts are swallowed and arrive at the small intestine; alkaline pH and digestive juices stimulate cysts to release 4 trophozoites Trophozoites attach, multiply, actively move about and feed Pathogenesis of Amebiasis Asymptomatic in 90% of patients Ameba may secrete enzymes that dissolve tissues and penetrate deeper layers of the mucosa Causing dysentery, abdominal pain, fever, diarrhea, and weight loss Carried by 10% of world population Life-threatening manifestations are: hemorrhage, perforation, appendicitis, and tumor-like growths, 3 amoebomas Severe forms 10% fatality rate Initial diagnosis by microscopic examination of a fecal smear for presence of trophozoites or cysts Treatment - metronidazole, tinidazole, or paromomycin, or nitazoxanide Amoebic Infections of the Brain Caused by Naegleria fowleri and Acanthamoeba 'Brain-eating Amoebas' Ordinarily inhabit standing water (warmer) Primary acute meningoencephalitis is acquired through nasal contact with water Make their way into the brain through the cribifrom plate in upper nasal cavity Infiltration of brain is usually fatal Acanthamoeba Infections of the Eye Caused by Acanthamoeba Resistant to chlorine Inhabit fresh water, can be in city water systems Can contaminate contact lenses when 'sink water' is used to clean/store lenses Can also be introduced by traumatic eye injury Eat away the cornea Permanent eye damage, and/or blindness
23.23)Describe the relationship between arthropod vectors and the parasites they carry. Define ectoparasites.
Arthropods - exoskeleton and jointed legs; includes arachnids and crustaceans; many must feed on blood and tissue fluid of host during life cycle; ectoparasites Those of medical importance transmit infectious microbes in the process of feeding - biological vectors TABLE 23.6 Common Arthropods That Are Biological Vectors of Disease Arthropod Vectors: Insects Mosquitoes - require an aquatic habitat Females take blood meal transmitting disease: malaria, filariasis, Dengue fever Fleas - highly mobile, flattened bodies; feed on warm-blooded animals Carry zoonotic diseases: plague, murine typhus 13 Lice - small, soft; attach to head and body hair feeding inconspicuously on blood and tissue fluid Release feces that contaminate wound; epidemic typhus, relapsing fever Arthropod Vectors: Insects (b) Flea (note blood in gut). Source: CDC (a) Mosquito feeding on human Source: James Gathany/CDC Arthropod Vectors: Insects (c) Louse. Source: James Gathany/CDC Arthropod Vectors: Arachnids Ticks - cling on vegetation and attach to host on contact; larvae, nymph, and adults get blood meal by piercing skin of host Hard or ixodid ticks - small compact, rigid bodies; transmit bacterial, rickettsial, and viral diseases Soft or argasid ticks - flexible outer bodies; transmit relapsing fever Arthropod Vectors: Arachnids (d) Hard tick, Amblyomma, A: shown actual size; B and C: feeding stages; D and E: stages of engorgement. Source: Dr. James Ochi/CDC-DPDx Arthropod Vectors: Arachnids (e) Soft tick, Ornithodoros, ventral and dorsal views. Source: Dr. James Ochi/CDC-DPDx
23.18)List the diseases associated with each of the common filarial worms.
Complete their life cycle in human blood, lymphatics, or skin Filarial worms; elongate, filamentous bodies, spread by biting arthropods Cause filariasis, a chronic, deforming disease The species responsible for most filariases are Wuchereria bancrofti - elephantiasis Onchocerca volvulus - river blindness Loa loa - eye worm
23.8) Describe the life cycle and the pathogenesis of Plasmodium malaria and the strategies used to combat the disease
Asexual phase - human Infected female mosquito injects asexual sporozoites which travel to the liver; then schizogony (asexual division) generates merozoites that enter circulation in 5-16 days depending on species Merozoites attach to and enter red blood cells, convert to trophozoites and multiply; red cell bursts releasing merozoites that differentiate into gametes 6 Sexual phase - mosquito Mosquito draws infected RBCs; gametes fertilize forming diploid cell which forms sporozoites in stomach Sporozoites lodge in salivary glands; available to infect human host Malaria Lifecycle Asexual phase. During the phase in humans, sporozoites enter a capillary with the saliva of a feeding mosquito. Exoerythrocytic phase. Sporozoites invade the cells of the liver and undergo division and release large numbers of merozoites. Erythrocytic phase. Merozoites enter the circulation and invade red blood cells. Infection gives rise to a prominent phase-the ring trophozoite-in blood cells. Shizogony of this form produces additional merozoites that burst out and continue the infection cycle. Gametocyte phase. Some of the red blood cells enter the sexual cycle by giving rise to male and female gametocytes, which infect another mosquito when it feeds on the blood. Sexual phase. Final development occurs in the mosquito. It involves complex stages that ultimately produce the infectious sporozoite. Pathogenesis of Malaria Symptoms include episodes of chills-fever-sweating, anemia, and organ enlargement Symptoms occur at 48-72 hour intervals as RBCs rupture; interval depends on species P. falciparum most malignant type; highest death rate in children Diagnosis by presence of trophozoite in RBCs, symptoms Increasing drug resistance Therapy is chloroquine, mefloquine Diagnosis and Control of Malaria Diagnosis: presence of trophozoite inside RBCs in blood smears, symptoms (recurring chills, fever, and sweating) Treatment: most current based on quinine drugs Chloroquine, when drug resistance is not a problem, malorone otherwise Artemisinin-based combination therapy (ACT) (coartem) Control: mosquito decline and human prophylaxis Vaccination
23.22) Describe the major characteristics of Taenia saginata, and Taenia solium
Beef tapeworm Very large, up to 2,000 proglottids Humans are the definitive host Taenia solium tapeworm scolex displaying four suckers and two rows of hooklets. Source: CDC Taenia saginata Animals are infected by grazing on land contaminated with human feces Infection occurs from eating raw beef in which the larval form has encysted In humans, larva attaches to the small intestine and becomes an adult Causes few symptoms; vague abdominal pain and nausea; proglottids in stool Taenia solium Pork tapeworm Infects humans through ingesting cysticerci (encysted larvae) or eggs In cysticercosis, eggs hatch in intestine, releasing tapeworm larva that migrate to all tissues and encyst Most damaging if they lodge in the heart, eye, or brain May cause seizures, psychiatric disturbances
23.13) Describe the four basic helminth transmission cycles.
Cycle A In cycle A, the worm develops in intestine; egg is released with feces into environment; eggs are ingested by new host and hatch in intestine (example: Ascaris). Four Different Helminth Lifecycles Cycle B In cycle B, the worms mature in intestine; eggs are released with feces; larvae hatch and develop in environment; infection occurs through skin penetration by larvae (example: hookworms). 9 Four Different Helminth Lifecycles Cycle C In cycle C, the adult matures in human intestine; eggs are released into environment; eggs are eaten by grazing animals; (intermediate host); larval forms encyst in tissue; humans (definitive host); eating animal flesh are infected (example: Taenia). Four Different Helminth Lifecycles Cycle D In cycle D, eggs are released from human; humans (definitive host) are infected through ingestion or direct penetration by larval phase (examples: Opisthorchis and Schistosoma).
23.3) Identify the three groups of amoebas generally seen as human pathogens
Entamoeba Naegleria Acanthamoeba
23.20) Identify the intermediate and definitive hosts in trematode infestation. List the major types of trematode examples with their hosts
FLUKES Flatworms with ovoid leaflike bodies Have digestive, excretory, neuromuscular, and reproductive systems Lack circulatory and respiratory systems Animals such as snails or fish are usually the intermediate hosts and humans are the definitive hosts Trematodes Blood Flukes: Schistosomes (Cycle D) Schistosomiasis - prominent parasitic disease Schistosoma mansoni, S. japonicum, S. haematobium Adult flukes live in humans who release eggs into water; early larva (miracidium) develops in freshwater snail into a 2nd larva (cercaria) Larva penetrates human skin and matures into the liver; adults migrate to intestine or bladder and shed eggs (chronic organ enlargement) Humans are definitive host Snails are intermediate host 12 Liver Flukes (Cycle D) Trematodes of zoonotic origin Opisthorchis (Clonorchis) sinensis Cycles between mammals and snails and fish Humans are infected by eating inadequately cooked fish containing cercariae Larvae crawl into bile duct, mature, and shed eggs into feces; snail are infected Fasciola hepatica Cycles between herbivores, snails, and aquatic plants Humans are infected by eating raw aquatic plants; fluke lodges in liver Lung Flukes (Cycle D) Paragonimus westermani - cycles between carnivorous animals, snails, and crustaceans; humans infected by eating undercooked crustaceans; intestinal worms migrate to lungs
23.21)Discuss the anatomical adaptation of tapeworms to their intestinal habitat
Flatworms Long, very thin, ribbonlike bodies composed of sacs (proglottids) and a scolex that grips the intestine Each proglottid is an independent unit adapted to absorbing food and making and releasing eggs Principal causes of of human taeniasis: Taenia saginata Taenia solium
23.10) Explain the occasional presence of Cryptosporidium in domestic water source
Infects a variety of mammals, birds, and reptiles Causes an intestinal disease called cryptosporidiosis Organism exists in tissue and oocyst phases 1990s - 370,000 cases in Milwaukee, WI, due to contaminated water; filtration required for removal Cryptosporidium: An Intestinal Pathogen Ingestion of oocysts gives rise to sporozoites that penetrate intestinal cells 7 Causes gastroenteritis, headache, sweating, vomiting, abdominal cramps, diarrhea AIDS patients may suffer chronic persistent diarrhea Treatment: No effective drugs
23.12) Recall the difference between intermediate hosts and definitive hosts
Intermediate host - larval development occurs Definitive host - adulthood mating occur
23.15) Identify the transmission cycle of each of the most common intestinal nematodes (Ascaris, Trichinella and hookworms).
Intestinal Nematodes Cycle A Ascaris lumbricoides Trichuris trichuria Enterobius vermicularis Cycle B The Hookworms Strongyloides stercoralis (Strongyloidiasis) Trichinella spiralis (Trichinellosis) Tissue Nematodes Filarial Nematodes Wuchereria bancrofti (Bancroftian Filariasis) Onchocerca volvulus (River Blindness) Dracunculus medinensis (Dracontiasis) Ascaris lumbricoides & Ascariasis A large intestinal roundworm Most cases in the U.S. occur in the southeastern states Indigenous to humans Ascaris spends its larval and adult stages in humans; release embryonic eggs in feces, and are spread to other humans; food, drink, or contaminated objects 10 Ingested eggs hatch into larvae and burrow through the intestine into circulation and travel to the lungs and pharynx and are swallowed Adult worms complete cycle in intestines and reproduce - 200,000 eggs/day Enterobius vermicularis & Pinworm Infection Pinworm or seatworm Enterobiasis most common worm disease of children in temperate zones Eggs are picked up from surroundings and swallowed After hatching in the small intestine, they develop into adults Anal itching occurs when mature females emerge from intestine to release eggs Self-inoculation is common Tape test Hookworms Characteristic curved ends and hooked mouths Necator americanus and Ancylostoma duodenale Humans shed eggs in feces, which hatch into filariform larvae and burrow into the skin of bare feet Larvae travel from blood to lungs, proceed up bronchi and throat and are swallowed Worms mature and reproduce in small intestine and complete the cycle May cause pneumonia, nausea, vomiting, cramps, and bloody diarrhea Blood loss is significant - anemia
23.9) Describe the pathogenesis of Toxoplasma gondii
Intracellular apicomplexan parasite with extensive distribution Lives naturally in cats that harbor oocysts in the GI tract Acquired by ingesting raw meats or substances contaminated by cat feces Most cases of toxoplasmosis go unnoticed except in fetus and AIDS patients who can suffer brain and heart damage Treatment: pyrimethamine and sulfadiazine
23.16) Summarize the mode of infestation and pathology of Trichinella
Larvae encysted in animal muscle are the infective stage. When ingested, cysts hatch and mature in the intestinal lining. Adults reproduce and form new larvae. These burrow through the intestine and enter the circulation. Larvae eventually form cysts in skeletal muscle that can remain for years. Circle shows a biopsy of human skeletal muscle infected with the coiled larvae of Trichinella spiralis.(next slide) Trichinella spiralis & Trichinellosis Life cycle entirely within mammalian host Pigs, Bears, cats, dogs, and rats serve as reservoirs
5.2) Discuss the importance of Kingdom Protista
Medically important pathogens Entamoeba histolytica Naegleria fowleri Giardia lamblia Plasmodium vivax 2 Toxoplasma gondii Pathogenic Protozoan Protozoans like Giardia intestinalis are routinely present in natural bodies of water and ingestion can cause severe disease.
23.1)Describe the three life cycles of the major protozoan pathogens.
Parasitology - The study of eukaryotic parasites: Protozoa (unicellular animal-like organisms) Helminths (worms) Cause 20% of all infectious diseases Less prevalent in industrialized countries; increasingly common in AIDS patients (LO1) Major Protozoan Pathogens Single-celled, animal-like microbes, most having some form of motility Estimated 100,000 species, approximately 25 are important pathogens Life cycles vary (1) Most propagate by simple asexual cell division of the active feeding cell (trophozoite) (2) Many undergo formation of a cyst (3) Others have a complex life cycle that includes asexual and sexual phase
23.6) State the importance of cysts to the transmission cycle of Giardia.
Pathogenic flagellate Unique symmetrical heart shape with concave ventral surface that acts like a suction cup 4 Cysts are small, compact, and multinucleate Reservoirs include beavers, cattle, coyotes, cats, and humans ('Beaver Fever') Cysts can survive for 2 months in environment Giardia intestinalis and Giardiasis Usually ingested with water and food ID 10 to 100 cysts Cysts enter duodenum, germinate, travel to jejunum to feed and multiply Causes giardiasis - diarrhea, abdominal pain Diagnosis is difficult because organism is shed in feces intermittently Treatment - Quinacrine or metronidazole Agent is killed by boiling, ozone, and iodine
23.14) Describe the strategies used to diagnose and control helminth infections.
Pathology arises from worms feeding on and migrating through tissues, accumulation of worms, and worm products Diagnosis: based on blood cell count (eosinophilia), serological tests; eggs, larvae, or adult worms in feces; sputum, urine, blood, or tissue biopsies Treatment: Antihelminthic drugs suppress a helminthic metabolic process that differs from the human process, inhibit the worm's movement, prevent it from holding position, and act locally in the intestine Nematode (Roundworm) Infestations Most abundant animal groups The vast majority are free- living soil and freshwater worms, but around 200 are parasitic (50 species affect humans) Elongated, cylindrical worms with protective cuticles, circular muscles, a complete digestive tract, and separate sexes The human parasites divided into: Intestinal nematodes: develop to some degree in the intestine Ascaris, Strongyloides, and hookworms Tissue nematodes: spend their larval and adult phases in various soft tissues other than the intestine (for example, various filarial worms)
5.1) Explain what is meant by the term protist, and outline the two types of organisms belonging to this designation
The term "protist" refers to any eukaryote that is not a fungus, animal or plant. *Fall into two basic categories: Algae - eukaryotic organisms, usually unicellular and colonial, that photosynthesize with chlorophyll Protozoa - unicellular eukaryotes that lack tissues and share similarities in cell structure, nutrition, life cycle, and biochemistry Algae Photosynthetic organisms Microscopic forms are unicellular, colonial, filamentous Macroscopic forms are colonial and multicellular Contain chloroplasts with chlorophyll and other pigments Cell wall - composition varies May or may not have flagella Algae Most are free-living in fresh and marine water - plankton Provide basis of food web in most aquatic habitats Produce large proportion of atmospheric O2 Dinoflagellates can cause red tides and give off toxins that cause food poisoning with neurological symptoms Thoughts on Algae Algae typically have few health risks associated with them but many industrial uses. The cell walls of the diatoms seen here are used to filter drinking water, and their photosynthetic activity is responsible for most of the oil on Earth. Protozoa Vary in shape, lack a cell wall Most are unicellular; colonies are rare Most are harmless, free-living in a moist habitat Some are animal parasites and can be spread by insect vectors All are - heterotrophic Lack chloroplasts Cytoplasm divided into ectoplasm and endoplasm Feed by engulfing other microbes and organic matter Protozoa Most have locomotor structures flagella, cilia, or pseudopods Exist as trophozoite-motile feeding stage Many enter a dormant stage during unfavorable conditions for growth and feeding-cyst All reproduce asexually, mitosis or multiple fission; many also reproduce sexually- conjugation
5.4) Explain the elements of helminth biology, life cycles, parasitic nature, and reproductio
Three stages of Helminthic lifecycle: Fertilized egg (embryo) Larval stage Adult stage In most, adults derive nutrients and reproduce sexually in a host's body Must complete the life cycle by transmitting an infective form to the body of another host Intermediate host - larval development occurs Definitive host - adulthood mating occurs 8 Transport host - intermediate host that experiences no parasitic development A Helminth Cycle: The Pinworm Person swallows microscopic eggs Picked up from another infected person by direct contact Or by touching articles an infected person has touched Eggs hatch in the intestine Release larvae that mature in to adult worms (about 1 month) Male and female worms mate Female migrates out of the anus to deposit eggs Causes intense itchiness Relieved by scratching Scratching contaminates the fingers which transfer the eggs Eggs spread to others or the original host re-infects him or herself Lifecycle of the Pinworm Pathogenic Helminths Source: CDC Parasitic worms like this pork tapeworm (Taenia solium) use a combination of hooks and suckers to attach to the intestinal wall of their host. These infestations are exceedingly difficult to treat because the worm's size and biology makes it more like its human host than an invading microbial organism.
23.19) Describe the signs and symptoms associated with Wucherereia bancrofti nematode
Tropical infection spread by mosquitoes Vector deposits larvae which move into lymphatics and develop into adults Chronic infection causes blockage of lymphatic circulation and elephantiasis, massive swelling in the extremities Onchocerca volvulus & River Blindness Transmitted by biting black flies Larvae develop into adults in subcutaneous tissues Adult females migrate via the blood to the eyes, provoking inflammatory reactions Coinfection with Wolbachia bacteria causes river blindness Treatment: tetracycline and ivermectin Loa loa: The African Eye Worm Spread by bite of small flies Temperature-sensitive worm migrates around/under the skin and may enter the eye Treatment - pull worm from a small hole in conjunctiva or diethylcarbamazine Dracunculus medinensis & Dracontiasis Dracontiasis, caused by the guinea worm, affects residents of India, the Middle East, and Central Africa. The parasite is carried by Cyclops, a microscopic arthropod commonly found in still water After ingesting water from a contaminated water source, the larvae are freed from their Cyclops host and penetrate into subcutaneous tissues of the intestine, where they form highly irritating lesions After efforts from WHO and Carter Foundation, cases have dropped from 3.5 million in 1986 to 24 cases in 2015 and will soon be eliminated from the earth
5.3) Describe the major groups of helminths and their basic morphology and classification
Worms are NOT microorganisms Multicellular animals, organs for reproduction, digestion, movement, protection Parasitize host tissues Have mouthparts for attachment to or digestion of host tissues Most have well-developed sex organs that produce eggs and sperm Fertilized eggs go through larval period in or out of host body Major Groups of Parasitic Helminths Flatworms (Platyhelminthes) flat, no definite body cavity; digestive tract a blind pouch; simple excretory and nervous systems Cestodes (tapeworms) Trematodes or flukes, are flattened, non-segmented worms with sucking mouthparts Parasitic Flatworms A Cestode A Tremetode Major Groups of Parasitic Helminths Roundworms (Nematodes) round, a complete digestive tract, a protective surface cuticle, spines and hooks on mouth; excretory and nervous systems poorly developed Distribution and Importance of Parasitic Worms Approximately 50 species parasitize humans Distributed worldwide; some restricted to certain geographic regions with higher incidence in tropics Acquired through ingestion of larvae or eggs in food; from soil or water; some are carried by insect vectors Afflict billions of humans
