The Parasite: Cryptosporidiosis is caused by the Cryptosporidium parasite, and both the disease and parasite are commonly referred to as "crypto." The crypto parasite is found in every region of the United States and throughout the world. Its life cycle begins when it is expelled through the feces of its current host. In this "spore" or "oocyst" phase the parasite can survive for long periods of time -- up to several months -- outside of a host, thanks to its thick outer shell or "cyst." This shell also makes the parasite highly resistant to chlorine, which is frequently used to purify water.

The parasite generally makes its way to the next unsuspecting victim via drinking or recreational water that has been contaminated with human or animal feces. Once ingested (or occasionally inhaled) by a new host, the crytpo parasite excysts (or sheds its shell) in the small intestine and proceeds to reproduce asexually. This leads to two types of crypto parasites in the oocyst state: thin-walled parasites that will remain in the host's body and restart the infection process and thick-walled oocysts that will be excreted and go on to infect a new host.