Cryptosporidiosis is caused by the protozoan Cryptosporidium. These small spherules inhabit the microvillus border of the intestinal epithelium. There, the protozoa shed infected oocysts into the intestinal lumen, where they pass into the feces.

Cryptosporidiosis is a zoonosis in which infection from cattle, goats, turkeys, and other farm animals can be transmitted to humans. These oocysts are particularly hardy, resisting destruction by routine water chlorination. This increases the risk of infection spreading through contact with contaminated water. The disease can also be transmitted by means of contaminated food and person-to-person contact.

Complications can be particularly severe in immunocompromised patients. In these patients, profuse, watery diarrhea can lead to severe fluid and electrolyte depletion and malnutrition. Rectal excoriation and breakdown can also result.

If the biliary tract becomes affected, papillary stenosis, sclerosing cholangitis, or an acalculous cholecystitis can occur.

A history of foreign travel or illness in other family members will usually correlate with symptom onset. Although asymptomatic infections can occur in both normal and immunocompromised patients, the typical patient with cryptosporidiosis develops symptoms after an incubation period of 5 to 21 days; however, the onset may be explosive. (The incubation period may be shorter in an immunocompromised patient.) The patient initially complains of watery, nonbloody diarrhea. Other signs include abdominal pain, anorexia, nausea, fever, flatulence and bloating, and weight loss. In the 10% of patients who develop biliary tract involvement, right upper abdominal pain may be severe. Signs and symptoms usually subside within 2 weeks but may recur sporadically for months to years.