Cryptosporidiosis is a disease affecting the intestines of mammals that is caused by Cryptosporidium, a protozoan parasite in the phylum Apicomplexa. It is a disease spread through the fecal-oral route; the main symptom is self-limiting diarrhea in people with intact immune system. In immunocompromised individuals, such as AIDS patients, infection can cause permanent & life-threatening diarrhea. Despite not being identified until 1976 it is one of the most common waterborne diseases and is found worldwide. The disease and parasite name are sometimes abbreviated to just Crypto.

Infection is through contaminated material such as earth, water, uncooked or cross-contaminated food that has been in contact with the feces of an infected individual or animal. Contact must then be transferred to the mouth and swallowed. It is especially prevalent amongst those in regular contact with bodies of fresh water whether through work or recreation.

Symptoms appear from two to ten days after infection and last for up to two weeks or so. As well as watery diarrhea there is often stomach pains or cramps and a low fever. Some individuals are asymptomatic (have no symptoms) but are still infective and thus can pass on the infection to others. Even after symptoms have finally subsided that individual is still infective for some weeks. There is no cure for the disease and treatment is limited to palliative care (focusing on treatment of symptoms), especially the replacement of lost fluids.

A number of species of Cryptosporidium infect mammals. In humans the main causes of disease are C. parvum and C. hominis (previously C. parvum genotype 1). C. canis, C. felis, C. meleagridis, and C. muris can also cause disease in humans.

Cryptosporidium has a spore phase (oocyst ) and in this state can survive for lengthy periods outside a host and also resist many common disinfectants, notably chlorine based disinfectants. Because of this resistance, water treatment to eliminate Cryptosporidium generally relies upon filtration or boiling. Recently, it has been discovered that Cryptosporidium ios sensitive to ultraviolet light, and water treatment based on this is being developed.