the number of enteric bacterial illnesses, the case-fatality rate reported annually from all cases, and the percentage of illnesses attributed to contaminated water supplies, which ranges from 3 to 75 percent. Enteric bacteria caused 14 percent of all waterborne disease outbreaks in the United States from 1970 to 1990 (Craun, 1991).

The enteric protozoan parasites produce cysts or oocysts that aid in their survival in wastewater. Important pathogenic protozoa include Giardia lamblia, Cryptosporidium parvum, and Entamoeba histolytica. (Helminth ova are present in untreated wastewater; however, they are relatively large and tend to drop out of effluent after primary and secondary treatment.) Waterborne outbreaks of amebic dysentery, caused by Entamoeba, have not been reported in the United States in over 15 years (Bennett et al., 1987). Giardia is recognized as the most common protozoan infection in the United States and remains a major public health concern (Craun, 1986; Kappus et al., 1992). The reported incidence of waterborne giardiasis has increased in the United States since 1971 (Craun, 1986). An average of 60,000 cases are reported annually, and 60 percent are estimated to be waterborne (Bennett et al., 1987). Because Giardia is endemic in wild and domestic animals, infection can result from water supplies that have no wastewater contribution. Densities of Giardia cysts in untreated wastewater have been reported as high as 3375 per liter (Sykora et al., 1991).