Cryptosporidiosis is a disease caused by the protozoan parasite Cryptosporidium. The species of the organism that affects mammals most commonly is Cryptosporidium parvum. A number of mammalian species including rodents, calves, dogs, cats and people can develop gastrointestinal tract disease due to infection with the organism. A high prevalence of serum antibodies to the organism in many dogs, suggests that exposure to the parasite is common.

The infective form of the organism is the oocyst, which is spread via fecal contamination of food or drinking water. The organism is very infective. It only takes a few oocysts to cause disease in people.

Cryptosporidiosis may be a primary disease, or it may be a secondary disease in dogs with weakened immune systems. The risk of exposure increases in crowded or unsanitary conditions. In dogs, it is more commonly seen in young animals less than six months old.

Typically, cryptosporidiosis is a self-limiting disease in dogs with competent immune systems; many dogs can be infected without showing clinical signs at all. Others will have mild diarrhea, but recover uneventfully. Even though young dogs are more susceptible to becoming infected, they too may never develop clinical signs. The signs of cryptosporidiosis include acute onset of lethargy, abdominal cramps and profuse watery diarrhea. The illness generally subsides without treatment, although persistent diarrhea and dehydration occasionally develops. The severity of the disease depends on the immune competence of the dog. Immunocompromised people, such as those with HIV/AIDS, may suffer severe diarrhea that never resolves and may even prove fatal.