Cryptosporidium is a microscopic parasite that can be found in soil, food, and water, or on surfaces that have been contaminated with feces from infected humans or animals. Transmission occurs by the fecal-oral route and infection causes the diarrheal disease cryptosporidiosis. The disease is usually mild and self-limiting (1–2 weeks) but in people with impaired immunity, the illness can be serious, prolonged, and life-threatening. Diagnosis relies on the detection of oocytes in fecal or environmental samples either by microscopy or monoclonal antibody ELISA.