Apicomplexan parasites are intracellular pathogens that invade host cells using an organelle termed the apical complex, and are usually transmitted among hosts via an invertebrate vector or an intermediate host. Cryptosporidium species have a simpler life cycle than other apicomplexans, complete their life cycles in the intestinal tract of a single host and can be transmitted to other individuals through ingestion of oocysts, the stage of the parasite excreted in the feces. Significantly, no insect vector or intermediate host is required. Bovines play a central role in maintaining and disseminating C. parvum oocysts due to their susceptibility for this infection and the large number of oocysts excreted during diarrheal episodes.

Infection is thought to occur primarily through ingestion of water contaminated with oocysts or by fecal-oral transmission. The infective oocyst stage is often detected in surface water and in public water supplies. Although the human disease is usually self-limiting, causing acute diarrhea in the general population, it is life threatening for immuno-compromised individuals and disease presents a very serious challenge for people with AIDS. Several very significant outbreaks of cryptosporidiosis caused by contaminated drinking or recreational water have been documented, emphasizing the importance of this pathogen to public health. Significantly, over 400,000 cases of cryptosporidiosis were reported in an outbreak in Milwaukee in 1993. Over one hundred people died in that outbreak. Other significant outbreaks have occurred in both developing and developed parts of the world.

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