Pathogens, such as Giardia, Cryptosporidium and Legionella, are often found in water. If consumed, these pathogens can cause gastrointestinal illness (e.g., diarrhea, vomiting, cramps) and other health risks. These illnesses may be severe and sometimes fatal for people with weakened immune systems. Cryptosporidium is a significant concern in drinking water because it is resistant to chlorine and other disinfectants. The Surface Water Treatment Rules were established to protect against these pathogens. To protect public health, drinking water from lakes, rivers streams and some other sources needs to be treated. This treatment includes disinfection and, in most cases, filtration.
EPA Giardia/ Cryptosporidium info