Legionellosis is a serious lung infection caused by a bacterium called Legionella pneumophila. This bacterium was discovered following an outbreak of pneumonia that affected several participants at a convention of American Legion veterans held at a hotel in Philadelphia in 1976. Found in the natural environment, it proliferates in artificial water systems when conditions for its growth are met, particularly between 25 and 45°C. Infection occurs via the respiratory tract, through inhalation of contaminated water dispersed as an aerosol. To date, no human-to-human transmission has been demonstrated. The incubation period is generally 2 to 10 days. Legionellosis primarily affects adults and particularly impacts individuals with risk factors such as advanced age, smoking, chronic respiratory diseases, diabetes, immunosuppressive treatments, or alcoholism. Clinical symptoms begin with a cough that is initially non-productive and a febrile flu-like illness that rapidly worsens, leading to severe pneumonia requiring hospitalization in the vast majority of cases.