Cryptosporidium parvum is a tiny yet insidious waterborne parasite that wreaks havoc worldwide. This parasite is a major cause of diarrhea and malnutrition in small children in developing countries, and causes severe disease in AIDS and other immune compromised patients in the developed world. Cryptosporidium is resistant to water chlorination and has caused massive outbreaks in the U.S., which has led to the concern that the parasite could be used as a bio-terrorism agent. There are neither vaccines nor effective drugs available to respond to these multiple threats to human health.