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Cryptosporidiasis is a relatively mild GI infection caused by the coccidian protozoan Cryptosporidium parvum (parvum means small). Its oocysts are only 4 - 6 um in size and escape many flltration systems; they are also resistant to chlorine and ozone and can thus contaminate water supplies. Cryptosporidium is infamous for an outbreak in Milwaukee that sickened 300,000 people. It has an animal reservoir, putting veterinarians at higher risk. This infection is very serious in AIDS patients, causing up to 50 stools per day.