Legionellosis is a lung infection caused by a bacterium called Legionella. This bacterium is present in the natural environment and can proliferate in artificial water systems when conditions for its growth are met, particularly between 25 and 45°C: hot water systems, cooling towers, and other facilities (hot tubs, spa baths, humidifiers, decorative fountains, aerosols, etc.). Contamination occurs via the respiratory tract, through inhalation of contaminated water dispersed as an aerosol. To date, no human-to-human transmission has been reported. In France, surveillance of Legionnaires’ disease relies on data collected through mandatory reporting (DO). All cases of Legionnaires' disease must be reported to the Regional Health Agency (ARS). Upon receipt of the mandatory report, ARS staff verify the information and conduct an investigation to identify risk exposures, search for other cases linked to these exposures, and implement control and prevention measures as necessary.