Cryptosporidiosis is an intestinal infection caused by ingestion of the infective oocysts of Cryptosporidium species.

Transmission is primary by the fecal-oral route. Cryptosporidiosis is more common in conditions of crowding and poor sanitation, and infection is a major cause of diarrhea in children in developing countries. In developing countries, many adults may asymptomatically excrete oocysts, and oocysts can frequently be found on vegetables and can contaminate the water supply. In the United States, outbreaks have been associated with contaminated drinking water, contaminated recreational water (including lakes and public pools), and contaminated food.

The incubation period is usually 7-10 days (range 1-30 days).

Clinically, symptoms for cryptosporidiosis can vary widely. In immunocompetent hosts, diarrhea is usually self-limited (lasting 1-2 weeks). The diarrhea may be mild, or it may be voluminous and watery.

In immunocompromised hosts, including patients with AIDS, infection can cause devastating illness. The diarrhea can be severe and prolonged (lasting months), leading to dehydration. In this case, the illness could be life-threatening, and some patients may require parenteral nutrition.

Elderly patients typically have more severe disease.

In addition to watery diarrhea, patients may also complain of nausea, vomiting, fever, and abdominal cramping. In immunocompromised patients, infection can also involve the biliary or respiratory tract.