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Background: Problems, such as spoor supervision, lack of disinfection, poor quality control, continue to jeopardize drinking water safety in rural China.On May 7, 2011, a Shigella outbreak was reported in Town X, Guizhou Province.We investigated this outbreak to identify the mode of transmission and to develop control measures.Methods: We defined a probable case as onset of diarrhea (≥3 times/24h) plus mucopurulent bloody stool or tenesmus among residents of this town during April 1-May 20.A confirmed case was a probable case plus positive Shigella culture from the stools.In a case-control investigation, we compared exposures to water, food and hygienic habits between 137 probable or confirmed case-patients and 137 asymptomatic controls, frequency-matched by age.We evaluated the water supply and cultured stool and tap-water specimens for Shigella.Results: 0.63% (317/49934) of the villagers developed illness.The attack rate of villages using tap-water was 3.3% compared to 0.6% of those not using tap-water.66% (90/137) of case-patients and 16% (11/137) of control-persons drank untreated tap-water (OR=9.8, 95% CI: 5.3-18).The OR increased 1.7 times (95% CI: 1.4-2.0, Logistic regression) per cup per day of unboiled tap water consumed.Environmental investigation found domestic and agricultural sewage being discharged into the upper reach of the tap-water source, and the Water Works supplied the water to users without sterilization.The total bacterial count in the tap water was >3100cfu/ml, and E.coli count was >1600MPN/m1.5 stool specimens were culture positive for Shigella flexneri.Conclusions: Untreated drinking water from the Water Works, polluted with Shigella flexneri 2b caused this outbreak.We recommend chlorination of drinking water, and improved quality control for small central water supplies for rural areas.