Cryptosporidium is a protozoan parasite commonly found in surface waters (e.g., lakes and streams) especially where there is contamination from sewage or animal wastes. There are several types of Cryptosporidium, only one of which is known to cause disease in humans. Cryptosporidium forms protective oocysts that are very difficult to kill with disinfection. The parasites may be spread through unwashed hands, contact with contaminated surfaces, contaminated food or drinking water, or bathing in contaminated recreational waters.

Symptoms of Cryptosporidium ingestion includes diarrhea, nausea, and stomach cramps. These symptoms may be present with many other diseases and a special laboratory test is required to confirm the presence of Cryptosporidium. People with weakened immune symptoms are likely to have more severe and longer-lasting symptoms.