Cryptosporidium is a microscopic protozoan parasite found in surface waters such as rivers and lakes that are used as drinking water supplies. A number of waterborne disease outbreaks in the U.S. and other countries have been attributed to Cryptosporidium, including a 1993 outbreak of over 400,000 cases in Milwaukee, Wisconsin.
Like Cryptosporidium, Giardia is a protozoan parasite found in surface waters. Giardia can cause gastrointestinal disease. UnlikeCryptosporidium infections, it is treatable with chemotherapy. Interpretation of Cryptosporidium and Giardia positive results is difficult since the protozoans detected may be dead, the Cryptosporidium or Giardia detected may not be infective to humans, or the monitoring recoveries of the parasites are low. Only intact cysts or oocysts have the potential to be infective, however, the presence of even an empty Cryptosporidium or Giardia indicates infiltration into the water supply.