Cryptosporidium parvum is one of several species that cause cryptosporidiosis, a parasitic disease of the mammalian intestinal tract.

Extra-intestinal sites include the lung, liver, and gall bladder, where it causes respiratory cryptosporidiosis, hepatitis, and cholecystitis, respectively.

In immunocompromised patients, such as those with AIDS or those undergoing immunosuppressive therapy, infection may not be self-limiting, leading to dehydration and, in severe cases, death.

The protozoa also caused the largest waterborne-disease outbreak ever documented in the United States, making 403,000 people ill in Milwaukee, Wisconsin, in 1993.

[5] Those visiting areas, such as petting zoos, where they might access affected animals should ensure good hygiene measures such as hand washing.

[6] The C. parvum oocysts are incredibly durable, which can cause extended problems when attempting to control the spread of the parasite.

The oocyst can tolerate temperatures as low as −22 °C and for long periods of time, which means fecal contamination is possible even after going through deep freezing.

The oocysts can also tolerate shifts in pH that are found in some water treatment processes, and careful attention to detail must be done to prevent the possibility of infection.

[citation needed] Cryptosporidium parvum possesses numerous surface glycoproteins thought to play a role in pathogenesis.

In vitro, hyperimmune sera, as well as antibodies directed at specific epitopes on the GP900 protein, inhibit the invasion of C. parvum sporozoites into MDCK cell monolayers.