Typical Protozoan Pathogens
Protozoan infections are categorized by the type of motility, if any, and the complexity of their life cycles. All protozoans propagate via their trophozoite, or active feeding cell. The resting form, or cyst stage, enables them to exist outside their host. Major human parasite groups are found in the amoeba, the ciliate, the flagellate, and the sporozoan groups.

The most significant pathogenic ameba is Entamoeba histolytica, causative agent of amebic dysentery. It is transmitted through cyst-infected food and water. Humans are its only host. The trophozoites can cause ulcerations in the bowel, which can lead to perforation and infection of other peritoneal organs.

Naegleria fowleri and Acanthamoeba are free-living amoebae that infect humans exposed to contaminated water. Naegleria invades the brain through the nasal mucosa. Acanthamoeba invades through abraded skin or conjunctiva. CNS infections progress rapidly and are usually fatal.

Balantidium coli, the largest protozoan pathogen, is also the only ciliate pathogenic to humans. Cysts enter the host through contaminated water. Trophozoites colonize the intestinal mucosa and cause moderate diarrhea.

The flagellated protozoans contain several major pathogens: Trichomonas, Giardia, Trypanosoma, and Leishmania. The most important trichomonad causes an STD called trichomoniasis. Giardia flagellates causes a non-invasive infection of the intestinal mucosa, which can proceed to a severe, chronic form in some cases.

The hemoflagellates are systemic invaders that cause debilitating, life-threatening infections of lymphatics, blood vessels, and specific organs. All pathogens in this group are transmitted by arthropod vectors. All have several morphologically distinctive stages to their life cycles.

African sleeping sickness is caused by Trypanosoma brucei, which invades the blood, the lymphatics, and the brain. Tsetse flies are the vector and secondary host. T. cruzi is the causative agent of Chagas' disease. The reduviid beetle infects the host through its feces deposited at the bite region. Chagas' disease damages the heart and intestine in particular. Blood transfusion, sexual intercourse, and transplacental infection are also routes of infection.

Leishmania is the causative agent of leishmaniasis, a zoonosis transmitted by female sand flies which are also the secondary host. Macrophages transmit the infection throughout the human host. The migration of the infected macrophage determines whether the infection is cutaneous or systemic. Diagnosis is difficult because the infective form closely resembles certain bacteria and fungi.

The sporozoans are obligate intracellular parasites that lack motility in the mature state. Plasmodium and Babesia cause serious systemic infections transmitted by mosquitoes and ticks. Toxoplasma causes mild, systemic infections in healthy hosts. Cryptosporidium causes intestinal infections.

Plasmodium species are the causative agents of malaria, the major protozoan disease worldwide. The Plasmodium life cycle is timed to coincide with the reproductive cycle of female Anopheles mosquitoes, which require human blood for the development of their eggs. They transmit plasmodium sporozoites, which complete their development in the liver, invade red blood cells as merozoites, which then differentiate into reproductive gametes. These must be ingested by another mosquito in which sexual reproduction occurs. This gives rise to more sporozoites, continuing the cycle when the mosquito feeds again. Symptoms are caused by the damage to red blood cells. Prevention consists of mosquito abatement procedures and human prophylaxis. Quinine derivatives are the treatment of choice, but resistant strains are increasing.

Toxoplasma gondii causes a mild, systemic infection of healthy hosts, but fatal infections can occur transplacentally and in the immunosuppressed. Human are accidental hosts of Toxoplasma, a zoonosis of cats, pigs, and other domestic animals.

Cryptosporidium species cause an intestinal infection in both domestic animals and humans. Infection occurs through contact with infected animal feces, primarily through water supplies contaminated with cysts. Healthy hosts recover spontaneously, but the infection is potentially fatal to immunosuppressed hosts. There is currently no treatment.

A Survey of Helminth Parasites
Helminths are macroscopic, multicellular worms which have microscopic infective forms; eggs and encysted or free-living larvae. Two major divisions of helminths are the nematodes (roundworms), and platyhelminthes (flatworms). The flatworms are further divided into trematodes (flukes) and cestodes (tapeworms).

Most parasitic helminths have at least two hosts: the primary host, where sexual reproduction occurs, and the secondary host, which is invaded by immature or larval forms. There are five basic patterns of helminth life cycles characterized by the type of infection form, route of transmission, portal of entry, site of reproduction, portal of exit, and exiting form. Chemotherapy usually involves drugs that selectively inhibit worm metabolism or weaken their attachment. Preventive measures strive to interrupt the cycle of infection through improved public health conditions.

Nematodes of the type A life cycle enter the host as eggs, complete their life cycle in the host, and are transmitted to the environment in new eggs. Examples of cycle A nematodes include Ascaris, Trichuris, and Enterobius. Cycle B nematodes enter the host through the skin as larvae. They migrate from the blood to the lungs to the intestine, where they mature and reproduce. Excreted eggs hatch in the environment, producing more larvae. Examples of cycle B nematodes include hookworms Necator and Ancylostoma, the threadworm Strongyloides, and Trichinella, causative agent of trichinosis.

Nematodes, or roundworms, cause the most severe helminth infestations. They are grouped according to the host site of reproduction as either tissue or intestinal nematodes.

Ascaris lumbricoides, the largest and most common nematode, is the causative agent of ascariasis, an intestinal infestation in which both larvae and adults can invade other body tissues.

Trichuris trichura, a tropical nematode, is the causative agent of whipworm infection, an intestinal infestation characterized by ulceration and hemorrhage of the colon.

Enterobius vermicularis, a common nematode in temperate zones, is the causative agent of pinworm infection, an infestation limited to the intestine characterized by the appearance of egg-laying females around the anus.

Tissue nematodes include the filarial parasites Wuchereria, Onchocerca, and Dracunculus, all of which cause invasive, debilitating, life-threatening infestations among tropical and subtropical populations.

Trematodes exhibit the type D cycle of infection whereby cercarial larvae invade the human host and reproduce in specific organs. Eggs released in the urine or feces hatch into miricidal larvae that invade one or more secondary hosts.