Dehydration and malnutrition are the most common effects of cryptosporidiosis, an infection of the intestine by microscopic parasites known as Cryptosporidia.

First identified in 1976 as a cause of disease in humans, Cryptosporidia are normally passed in the feces of infected persons and animals in the form of cysts. The cysts can remain in the ground and water for months, and when ingested produce symptoms after maturing in the intestine and the bile ducts. The most common sources of infection are other humans, water supplies, or reservoirs. These are contaminated by animals that defecate in these areas. An outbreak in Milwaukee in 1993 in which over 400,000 persons were affected was traced to the city's water supply. Cysts of Cryptosporidia are extremely resistant to disinfectants commonly used in water treatment plants and are incompletely removed by filtration.

Most persons who experience significant symptoms have altered immune systems and diseases such as...