Toxoplasmosis. Toxoplasmosis is a protozoal disease caused by the sporozoan Toxoplasma gondii. This protozoan is transmitted from domestic house cats, usually by contact with their urine or feces. In humans, the protozoa multiply in the bloodstream and undergo a complex reproductive cycle. Patients experience fever, with other constitutional abnormalities, but symptoms are generally mild. However, in a pregnant woman, the protozoa may pass to the unborn fetus and cause tissue destruction. Also, in AIDS patients, toxoplasmosis can result in seizures and then brain inflammation, and it may be a cause of death.
Malaria. Malaria is a blood disease due to many species of the genus Plasmodium.Plasmodia are a group of protozoa of the Sporozoa (Apicomplexa) group. The parasites are transmitted by mosquitoes belonging to the genus Anopheles. When they infect individuals, they invade the red blood cells in the merozoite form. Within the red blood cells, the protozoa undergo various stages of their life cycle, and eventually the red blood cells rupture to release large numbers of parasites. The toxic compounds released during the rupture cause the paroxysms of chills and fever that characterize malaria. Severe anemia results, and renewed infections take place in new red blood cells. Treatment is effective with drugs such as quinine, chloroquine, and primaquine. The mortality rate remains high, however, and malaria infects approximately 300 million people each year.
Schistosomiasis. Schistosomiasis is caused by a multicellular, parasitic flatworm known as a fluke. The responsible flukes include Schistosoma mansoni and other species. In water, these parasites live in snails, and they enter the body through the skin of an individual who walks or swims in the infected water. The parasites multiply and live within the bloodstream, where they interfere with the flow of blood and lymph and cause local tissue damage. Various chemotherapeutic drugs are available to treat the disease.