Organisms of the genus Cryptosporidium are small (2 to 6 μm, depending on stage of life cycle) coccidian parasites that invade and then replicate within the microvillous region of epithelial cells lining the digestive and respiratory organs of vertebrates. 15 These obligate intracellular protozoans were recognized and named over 80 years ago 6-8 and have remained until recently nothing more than a biomedical curiosity. Prior to 1980, infections with species of Cryptosporidium were considered rare in animals and in man they were thought to be the result of a little-known opportunistic pathogen of immune deficient individuals outside its normal host range. Beginning in 1982, our concept of these protozoan parasites changed to that of important, widespread causes of diarrheal illness in several mammalian hosts, especially calves, lambs, and humans. Several species of Cryptosporidium have also been recognized as primary pathogens causing intestinal and respiratory disease in chickens, turkeys, and quails.