On September 20, 2001, the Saône-et-Loire Departmental Directorate for Social Affairs (DDASS) was alerted by a call from SOS Médecin in Chalon-sur-Saône regarding a suspected case of food poisoning at a hotel in the town of Dracy-le-Fort. The investigation conducted that same day revealed that, for several days, there had been numerous other cases of gastroenteritis outside the establishment, as well as complaints about poor water quality. The hypothesis of contamination of the drinking water system was therefore considered the most likely. Water samples were collected, restrictions on the use of water for food preparation were announced to the public on the afternoon of the 20th, and a decision was made to increase chlorination of the system. On September 22, the results of these samples confirmed significant microbiological contamination. An epidemiological investigation was conducted by CIRE Centre Est in collaboration with the DDASS and the InVS. It included: - A survey of consultation activity (total and for gastroenteritis) among general practitioners practicing in the municipalities served by the same distribution unit to confirm the outbreak and determine its geographic extent - A retrospective cohort study of a group of hotel trainees to rule out or confirm a mass foodborne illness outbreak - A retrospective cohort study of the general population of Dracy-le-Fort to determine the role of tap water as a source of contamination. An environmental investigation and microbiological testing of stool samples and water from the water supply system were also conducted. The results of this study confirmed the occurrence of an outbreak of acute gastroenteritis caused by human-genotype Cryptosporidium and polymorphic bacteria following consumption of water from the water supply system, with a very high attack rate. The most likely hypothesis is that the system was contaminated due to a malfunctioning valve causing wastewater to backflow into the water supply system. (R.A.)