A total of 192 outbreaks of waterborne disease affecting 36,757 persons were reported in the United States during the period 1971‐1977. More outbreaks occurred in nonmunicipal‐water systems (70%) than municipal‐water systems; however, more illness (67%) resulted from outbreaks in municipal systems. Almost half of the outbreaks (49%) and illness (42%) were caused by either the use of untreated or inadequately treated ground water. An unusually large number of waterborne outbreaks affected travelers, campers, visitors to recreational areas, and restaurant patrons during the months of May‐August and involved nonmunicipal‐water systems which primarily depend on ground‐water sources. The major causes of outbreaks in municipal systems were contamination of the distribution system and treatment deficiencies which accounted for 68% of the outbreaks and 75% of the illness that occurred in municipal systems. Use of untreated ground water was responsible for only 10% of the municipal system outbreaks and 1% of the illness. The major cause of outbreaks in nonmunicipal systems was use of untreated ground water which accounted for 44% of the outbreaks and 44% of the illness in these systems. Treatment deficiencies, primarily inadequate and interrupted chlorination of ground‐water sources, were responsible for 34% of the outbreaks and 50% of the illness in nonmunicipal‐water systems. Copyright © 1979, Wiley Blackwell. All rights reserved
