Cryptosporidum is a protozoan, a single-celled parasite that lives in the intestines of humans and other animals. The organism causes an intestinal malady called cryptosporidiosis (which is commonly called "crypto").

The members of the genus Cryptosporidium infects epithelial cells, especially those that line the walls of the intestinal tract. One species, Cryptosporidium muris, infects laboratory tests species, such as rodents, but does not infect humans. Another species, Cryptosporidium parvum, infects a wide variety of mammals, including humans. Calculations have indicated that cattle alone release some five tons of the parasite each year in the United States alone.

Non-human mammals are the reservoir of the organism for humans. Typically, the organism is ingested when in water that has been contaminated with Cryptosporidium-containing feces. Often in an environment such as water, Cryptosporidium exists in a form that is analogous to a bacterial spore. In the case...