CHLORINE RESISTANCE IS EMERGING. “Pathogens such as Cryptosporidium are resistant to chlorination and can appear even in high quality water supplies. Cryptosporidium was the cause of the largest reported drinking water outbreak in U.S. history, affecting over 400,000 people in Milwaukee in April 1993. More than 100 deaths are attributed to this outbreak.”
http://www.waterandhealth.org/drinkingwater/wp.html