Cryptosporidiosis is caused by the protozoan Cryptosporidium. These small spherules inhabit the microvillus border of the intestinal epithelium. There, the protozoa shed infected oocysts into the intestinal lumen, where they pass into the stool. Millions of these parasites can be released in a single bowel movement from an infected human or animal. Cryptosporidium oocysts are particularly hardy, resisting destruction by routine water chlorination. This increases the risk of infection through contact with contaminated water. Cryptosporidiosis may also be found in soil or on surfaces that have been contaminated with the feces from infected humans or animals. The disease can also be transmitted via contaminated food and person-to-person contact.