Outbreaks in drinking-water systems, 1991-1998.

During 1991-1998, 126 outbreaks, 429,021 cases of illness, 653 hospitalizations, and 58 deaths were reported in public and individual water systems in 41 states and three U.S. territories. A bacterial, viral, or protozoan etiology was identified in 41 percent of the outbreaks, and a chemical contaminant was identified in 18 percent. No etiological agent was determined in the remaining outbreaks. Important causes of outbreaks included contamination of untreated groundwater, inadequate disinfection of groundwater, and distribution system deficiencies, especially cross-connections and corrosive water. The responsible pathogen or chemical was identified in water samples collected during 31 percent of the reported outbreaks. Coliform bacteria were detected in water samples collected during the investigation of infectious-disease outbreaks in 83 percent of noncommunity and 46 percent of community water systems, but very few of these systems had exceeded the U.S. Environmental Protection Agency's maximum limit for total coliforms in the 12 months before the outbreak.