On April 12, 2012, the Regional Health Agency (ARS) notified the Auvergne Regional Health Agency (Cire Auvergne) of a high level of microbial contamination in the water supply system of the municipalities of Pleaux and Barriac-les-Bosquets, in the Cantal department. Epidemiological and environmental investigations were launched to characterize the outbreak, confirm the waterborne origin, and determine the circumstances surrounding the contamination. A cohort study (n = 691 respondents, overall response rate of 39.7%) confirmed the association between the occurrence of acute gastroenteritis (AGE) and the consumption of tap water (RR = 1.69; 95% CI [1.27–2.27]) as well as an increase in this risk with the amount of water consumed (p<0.001). The attack rate was 27%, with significant differences depending on whether areas benefited from intermediate re-chlorination. The overall health impact of this outbreak was estimated at 477 cases of AGE among 1,786 people served by the water system. The main symptoms of AGE in affected individuals included diarrhea (89.2%), abdominal pain (87.1%), and vomiting (64%). The microbiological investigation revealed the presence of serogroup II norovirus in several patients, as well as in a water sample taken from the source after the outbreak. The environmental investigation revealed operational and monitoring failures at the water treatment plant and in the water distribution network. Analysis of health insurance reimbursement data for the 2009–2011 period also identified at least two other outbreaks of acute gastroenteritis in this area, which were very likely waterborne. (R.A.)