Cryptosporidium species are intracellular gastrointestinal parasites that were initially thought to cause disease only in animals. The parasite continues to be a major threat to human health for 2 major reasons. First, the current methods of water purification are ineffective for its removal from the public water supply. Second, there is no effective therapy for cryptosporidiosis. Although most cases of cryptosporidiosis in humans are self-limited, the consequences of infection can be devastating in immune compromised individuals and in children living in underdeveloped countries. Advances in our understanding of Cryptosporidium species have been limited because the organism can be propagated only in live animal hosts. Nevertheless, newer molecular techniques, such as PCR, have provided new insights into the complex epidemiology of this parasite.

Cryptosporidium infection is a zoonosis with mainly bovine and human reservoirs. Humans are thought to acquire the parasite by the ingestion of oocysts, which are shed in the stool of infected animals or other humans. In developing countries, cryptosporidiosis is endemic and is one of the most common causes of persistent diarrhea among children. In the developed world, cryptosporidiosis mainly occurs in sporadic outbreaks and epidemics. Immuno deficient people, particularly those with deficiencies in cell-mediated immunity, are especially susceptible to Cryptosporidium infection and experience the most-severe consequences of this illness.