Protozoa (Giardia and Cryptosporidium)

These are single-celled organisms that live as parasites in humans and other animals. They normally cause gastroenteritis. They form hard-coated cysts that are discharged in the feces of infected people. At the height of an infection, huge numbers ranging from two million to ten million cysts for every gram of feces may be excreted. Although they are most often transmitted through direct contact, water is commonly implicated; however, swimming pools have been involved as a source of infection more often than drinking water. Cysts cannot multiply in water, but can survive for long periods. The cysts can be removed from water by filtration, but they are relatively resistant (especially Cryptosporidium) to chlorine disinfection. They may be efficiently inactivated by exposure to UV light and ozone. Although they have been recognized as causing human health problems only since the 1960s, it is likely that these parasites have always been with us.