borne protozoan, rose as a major concern in the 1960s; rotavirus and Norwalk virus were associated with a large number of outbreaks beginning in the 1970s; and Cryptosporidium, also a protozoan, was first associated with waterborne outbreaks in the 1980s (Figure 3-1).

Much of the information on the etiology of waterborne disease comes from investigations of outbreaks by state and local health departments and from voluntary reporting by physicians to the surveillance program maintained by the Centers for Disease Control and Prevention (CDC) and the Environmental Protection Agency (EPA). When an outbreak occurs and waterborne pathogens are suspected, epidemiological studies are conducted to identify whether water is the vehicle of transmission. If possible, the etiologic agent is determined by detection in clinical specimens collected from outbreak victims. For gastrointestinal illness, routine stool examinations by hospital laboratories typically include culturing for Salmonella, Shigella, and Campylobacter bacteria. At the specific request of a physician, many laboratories can also test for rotavirus, Giardia, and Cryptosporidium. Nevertheless, no specific agent is identified in many outbreaks, leaving the cause classified only as acute gastrointestinal illness of unknown etiology (AGI). Before 1982, in fact, most waterborne outbreaks reported were listed as AGI. Poor collection of clinical and/or water samples and limitations of diagnostic techniques for many enteric pathogens can prevent accurate determination of the pathogen. Clinical symptoms suggest that many of the AGI outbreaks may be due to viral agents, such as Norwalk virus and related human caliciviruses.

Enteric bacteria are associated with human and animal feces and may be transmitted to humans through fecal-oral transmission routes. Most illnesses due to enteric bacteria cause acute diarrhea, and certain bacteria tend to produce particularly severe symptoms. As measured by hospitalization rates during waterborne disease outbreaks (that is, the percentage of illnesses requiring hospitalization), the most severe cases are due to Shigella (5.4 percent), Salmonella (4.1 percent), and pathogenic Escherichia coli (14 percent) (Gerba et al., 1994). There is now evidence suggesting that Campylobacter, Shigella, Salmonella, and Yersinia may also be associated with illness that causes arthritis in about 2.3 percent of cases (Smith et al., 1993).

Most E. coli are common, harmless bacteria found in the intestinal tracts of humans and animals, but some forms of E. coli are pathogenic and cause gastroenteritis. A particular strain, E. coli 0157:H7, is enterohemorrhagic (causes bloody diarrhea), and 2 to 7 percent of infections have resulted in hemolytic uremic syndrome (HUS), in which red blood