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Cryptosporidium life cycle.

Cryptosporidium are transmitted mainly through contact with contaminated water (eg, drinking water, swimming pools) and occasionally through contaminated food. 1. Sporulated oocysts are excreted by the infected host in stool and possibly other routes (eg, respiratory secretions). Oocysts are infective when excreted; thus, direct and immediate fecal-oral transmission is possible. 2–3 (a–k). Oocysts are ingested (or possibly inhaled) and excyst. Sporozoites are released and parasitize epithelial cells of the GI tract or other tissues (eg, respiratory tract). In these cells, the parasites transform into trophozoites and multiply asexually then sexually, producing microgamonts (male) and macrogamonts (female), which mate and produce oocysts. The oocysts sporulate in the infected host. Two types of oocysts are produced: thick-walled oocysts, which are commonly excreted by the host, and thin-walled oocysts, which are involved primarily in autoinfection.

Image from the Centers for Disease Control and Prevention Image Library.