On May 25, 2000, the manager of the SODEXHO restaurant at the Cité Administrative in Strasbourg and the Health and Environment Department of the Bas-Rhin DDASS noticed a suspicious taste and odor in the drinking water at the Cité Administrative on Rue du Maréchal Juin. The "medicinal or chemical" taste of the water raised suspicions of chemical contamination due to backflow. An analysis conducted that same day indicated the presence of toluene, tetrachloroethylene, and bromoform, though in quantities well below the levels that could cause public health problems according to WHO recommendations. On May 26, 2000, the DDASS learned from the Urban Community of Strasbourg that a 600-mm-diameter pipeline in the neighborhood had been put back into service on May 25, 2000, following several weeks of work. New samples were collected in the area for physicochemical and bacteriological analysis. In light of the non-compliant bacteriological results and based on the hydraulic characteristics of the network, a geographic area potentially affected by the contamination was identified, covering approximately 60,000 residents. For this area, recommendations against drinking tap water were issued starting at 7:00 p.m. on Saturday, May 27 (during the regional news broadcast). Flushing operations were carried out on Saturday, May 27, and Sunday, May 28, and disinfection of the network was stepped up. On Monday, May 29, the DDASS was alerted by the Centre 15 to an unusually high number of calls—about thirty—regarding abdominal pain or gastroenteritis over the weekend of May 27–28. It was in this context that the DDASS decided to engage CIREI Est to conduct an epidemiological study to determine a possible link between this apparent increase in cases of gastroenteritis and contamination of the water supply. To address this question, a case-control study was conducted.