On Thursday, October 18, 2001, the school physician at the Lycée professionnel de l'Aa in Saint-Omer reported an outbreak of allergic-type symptoms to the Departmental Directorate of Health and Social Affairs (DDASS). The first case appeared at 10 a.m. in a student in the 11th-grade Health and Social Care (CSS) program. Subsequently, several students and staff members experienced symptoms that led to the dispatch of an ambulance and the hospitalization of 19 students. The school was closed from the evening of Thursday, October 18, until the morning of Monday, October 22. This outbreak lasted from October 18 to 24, affecting 70 students and 5 staff members. The hypotheses considered included food poisoning, handling of an allergenic substance, a malicious act in a classroom or on a bus, and, finally, a psychogenic phenomenon. The epidemiological, biological, and environmental analyses conducted following this outbreak of allergic reactions did not allow for the formal identification of a mechanism capable of explaining the episode. The temporal association of a pollution episode with a secondary psychogenic phenomenon is a hypothesis that should not be ruled out. (adapted from the text)