Worms, Cysts, and Others
Last, (but not the least of our worries) are microscopic worms, parasites, and protozoa. The biggest offenders are giardia and cryptosporidia, which cause serious diarrhea, dehydration, and intestinal disorders - even death in people with compromised immune systems. Water experts estimate that over 63% of water problems in the Unites States today are directly caused by giardia and cryptosporidia. When the environment becomes inhospitable (in the presence of chlorine, for example), either of these parasites can go into the cystic form (hard and impermeable). The cyst is chlorine resistant and very hard to kill.
Municipal utilities cannot completely remove these cysts. They have been found in most major municipal water systems in the U.S. Milwaukee had a huge outbreak of cryptosporidia in 1993 that killed over 100 people. San Francisco has repeatedly tested positive for giardia in its chlorinated water that travels hundreds of miles from the Sierra Mountains.