genetic material (DNA or RNA) allows prolonged survival in the environment. There are more than 120 identified human enteric viruses. Some of the better described viruses include the enteroviruses (polio-, echo-, and coxsackieviruses), hepatitis A virus, rotavirus, and Norwalk virus. Most enteric viruses cause gastroenteritis or respiratory infections, but some may produce a range of diseases in humans, including encephalitis, neonatal disease, myocarditis, aseptic meningitis, and jaundice (Gerba et al., 1995, 1996; Wagenkneckt et al., 1991; see Table 3-1). Cases of poliovirus are low in the United States due to almost universal immunization. Table 3-3 shows the average number of viral illnesses that occur annually in the United States for the different enteric viral groups. No general estimates exist regarding the percentage of viral illnesses attributable to contaminated water supplies.

Norwalk and Norwalk-like viruses cause most waterborne viral diseases. Norwalk virus usually causes mild diarrhea that lasts on average for two days. A significant portion of the waterborne outbreaks reported as AGI are probably caused by Norwalk-like viruses that are not identified because of diagnostic limitations; Kaplan et al. (1982) suggested that such viruses may cause 23 percent of all waterborne outbreaks reported as AGI. From 1989 to 1992, contaminated drinking water was implicated in four outbreaks associated specifically with Norwalk-like viruses and hepatitis A virus (Herwaldt et al., 1992; Moore et al., 1993). During the same period, 37 waterborne outbreaks of AGI affected 15,769 people. In 85 percent of the outbreaks, the water quality met national drinking water standards for coliform bacteria.