Doctors in Millau, having noted an unusually high number of mumps cases since January 1995 and surprised by the significant proportion of cases among vaccinated individuals, alerted the Aveyron Departmental Directorate of Health and Social Affairs (DDASS). To assess the scale of the outbreak, describe it, and evaluate the vaccine’s effectiveness, the DDASS of Aveyron and the National Public Health Network decided to conduct an investigation, enlisting the help of the Health Promotion Service to carry out a survey in schools. (adapted from the text).