Cryptosporidium species are protozoan parasites that cause mainly enteric illnesses in humans and other animals. The mode of transmission is most commonly waterborne, but other sources of infection, including foodborne and person-to-person spread, have been documented. The environmental form of the parasite is resistant to most water purification methods, including chlorination. Cryptosporidium infection usually causes a self-limited diarrheal illness but can be life-threatening in immunocompromised individuals. There is no effective therapy for cryptosporidiosis.