On April 14, 2026, healthcare professionals in the town of Rousset alerted the Provence-Alpes-Côte d’Azur Regional Health Agency (ARS Paca) to what they considered a concerning increase in cases of acute gastroenteritis (AGE) among their patients. With no apparent link between the cases, the hypothesis of a waterborne origin was quickly favored, and environmental investigations were requested by the ARS Paca. A chlorination failure was identified in the municipality’s drinking water system by the Marseille Water Company (SEM), which immediately carried out emergency chlorination of the tanks. The ARS subsequently recommended restricting water use and distributing bottled water to the public.

Santé publique France Paca-Corse was contacted by the ARS Paca on April 16, 2026, to provide support and offered to conduct an epidemiological investigation aimed at characterizing the outbreak and identifying the source of contamination to guide management measures. Data on emergency department visits for GEA, derived from the SurSaUD® emergency and death surveillance system, are compared between residents of Rousset and residents of neighboring municipalities (Peynier, Fuveau, Trets, and Puyloubier). An epidemiological survey was launched on April 17, 2026, among residents via an online questionnaire accessible via a QR code displayed at local healthcare providers and on the Rousset town hall website.