Santé publique France was asked by the Grand Est Regional Health Agency (ARS) to conduct an epidemiological investigation to assess the impact on the health of the population of the municipality of Hermonville (Marne) of a fecal contamination of the drinking water in February 2021. A telephone survey was conducted among individuals who had reported feeling ill to the town hall. A person was considered exposed if they reported having consumed drinking water since February 5, 2021, and was classified as a case of acute gastroenteritis if they presented with diarrhea or vomiting. Among the 285 people surveyed, 281 were exposed. Of these, 271 (96%) exhibited symptoms, and 243 (89%) met the case definition, bringing the attack rate to 17.1% (243 cases / 1,422 residents). The median duration of symptoms was three days (1–25 days). Among those surveyed, 277 (97%) reported having been informed of the total restrictions on use effective February 9; however, 22 of them (8%) had consumed the water as drinking water after that date, 37 (13%) had used it for brushing their teeth, and 30 (11%) for washing uncooked food. The survey assessed the severity of symptoms as well as awareness and adherence to prevention messages.