In early September 2003, following reports to health authorities of an increase in cases of acute gastroenteritis (AGE) in a municipality in the Ain department, and of an anomaly in the drinking water supply system, a descriptive investigation was conducted jointly by the DDASS of Ain and the CIRE Rhône-Alpes Auvergne to confirm whether an outbreak was occurring and, if so, to determine the circumstances surrounding its occurrence. The proportion of AGE cases among outpatient visits and the characteristics of the cases were studied using data collected from general practitioners in the municipality supplied by the drinking water distribution system in question. Sales data for medications commonly prescribed for gastroenteritis were also collected from the two pharmacies in the municipality (as well as a control pharmacy). Microbiological analyses of patient stool samples, as well as water analyses from the drinking water supply network, were conducted. A quantitative risk assessment (QRA) was also conducted for Cryptosporidium based on the AFSSA reference document. Data collected from physicians and pharmacies in the municipality revealed an increase in the proportion of consultations for acute gastroenteritis (AGE) and sales of medications for its treatment between August 25 and September 13, 2003. 387 consultations for acute gastroenteritis were recorded between August 18 and September 20, 2003, including 350 between August 25 and September 13 (representing a rate of consultations for acute gastroenteritis of 17% and 20%, respectively, for each period). The case descriptions (clinical presentation, date of symptom onset, place of residence) are consistent with the presumed cause of the outbreak (microbiological contamination of the water supply). The impact of the outbreak was estimated at nearly 800 excess cases of acute gastroenteritis (in a population estimated at a maximum of 10,000 people). Seventy-six stool cultures were performed, with varying levels of pathogen detection, identifying, among other pathogens, Shigella, Giardia, and Cryptosporidium. Water analyses of both the distribution system and the source water revealed frequent contamination by Cryptosporidium, as well as by Giardia. The application of the EQR approach yielded estimates consistent with the data collected during the epidemic investigation and supports the role of Cryptosporidium. This investigation confirmed the existence of a WEA outbreak in the population of Divonne-les-Bains in late August and early September 2003, coinciding with microbiological contamination of the public water supply. Given the heterogeneity of the stool culture results, this study can only suggest a multi-pathogen GEA outbreak, which is consistent with the origin of the contamination of the public water supply system due to backflow from the wastewater treatment plant. (R.A.)