Cryptosporidium is an intracellular protozoan that is one of the most common parasitic enteric pathogens in humans. Infection is associated with gastrointestinal disease in sporadic self-limited outbreaks among immunocompetent hosts and chronic illness in immunosuppressed patients. Transmission occurs via spread from an infected person or animal, or from a fecally contaminated environment, such as a food or water source. Cryptosporidiosis is associated with a secretory diarrhea and with malabsorption. The parasite intracellular nature interferes with intestinal absorption and secretion. Infection can be asymptomatic, a mild diarrheal illness, or severe enteritis with or without biliary tract involvement. In immunocompetent hosts, illness usually spontaneously resolves without therapy, but among immunosuppressed host infection can be a chronic debilitating illness with wasting and persistent diarrhea. Diagnosis depends on enzyme immunoassay or by microscopic identification of oocysts in stool or tissue. Organisms may be present in duodenal aspirates, bile secretions, biopsy specimens from gastrointestinal tract, or respiratory secretions.