Cryptosporidiosis (often called "crypto") is a diarrheal disease caused by a one-celled parasite called Cryptosporidium parvum. Few people had heard of crypto until 1993, when over 400,000 people in Milwaukee, Wisconsin, became ill after their drinking water became contaminated with the parasite. Large outbreaks and isolated cases of crypto have been identified in Oregon as well. This has focused attention on determining and reducing the risk for cryptosporidiosis from community and municipal water supplies.

For persons with suppressed immune systems, for example persons with AIDS, some cancers, or recent organ transplants, the infection may persist indefinitely, and symptoms may be more severe.