EPA AND CDC WARNING. “in 1994 and 1995, 45 million Americans drank water from water systems that fell short of SDWA standards. Adding gravity to the situation … CDC and the EPA advised that people with weakened immune systems should consult with their doctors and consider boiling their drinking water to kill any cryptosporidium.”
http://www.nrdc.org/health/kids/ocar/chap7.asp