Cryptosporidiosis is a mammalian disease affecting the intestines caused by the protozoal parasite Cryptosporidium. It is a disease primarily spread through the oral-fecal route; the main symptom is diarrhea. Despite not being identified until 1976 it is one of the most common waterborne diseases and found worldwide. The disease and parasite name are sometimes abbreviated to just Crypto.

Infection is through contaminated material such as earth, water, or uncooked foods that have been in contact with the feces of an infected individual. Contact must then be transferred to the mouth and swallowed. It is especially prevelent 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 but still infective. 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, 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 bases.