Cryptosporidium is a microscopic parasite found in water contaminated by feces. It causes severe diarrhea and is resistant to chlorine, making it a concern in surface water sources.

Cryptosporidium enters water systems through contamination by infected animal or human feces. This hardy parasite is notoriously difficult to detect and remove due to its small size and resistance to chlorine disinfection. Major outbreaks have occurred in the past, most notably in Milwaukee in 1993. To combat this threat, many water treatment plants now use filtration systems designed to remove Cryptosporidium. Regular testing of surface water sources is crucial for early detection and prevention of contamination.