Cryptosporidium is common in many sources of drinking water, and elevated concentrations can cause outbreaks of a serious disease. In March and April of 1993, malfunction at the water treatment plant in Milwaukee, Wisconsin resulted in deaths and thousands of sick people. There is no effective treatment or cure. Most individual recover, but very young children and persons with impaired immune functions (e.g., patients getting kidney dialysis, HIV positive individual) have high risk of death.
Cryptosporidium is a single-cell protozoan. As part of its life cycle, an oocyst that is roughly spherical (3 to 6 µ m in diameter) is formed that is highly resistant to disinfectants such as chlorine. Determining whether water is safe to drink is based on counting the oocysts, but the specification is low. Samples must be concentrated or filtered before counting; the efficiency of recovery from the filters is low.
The source of information was Michael B. Kelly, "The removal of Cryptosporidium by selected
drinking water treatment processes", Ph.D. Thesis, Rensselaer Polytechnic Institute, June 1996