Legionnaires’ disease is a severe form of atypical pneumonia caused primarily by Legionella pneumophila. The disease was first identified following a 1976 outbreak at an American Legion convention in Philadelphia. It is transmitted through inhalation of contaminated aerosols from water sources such as cooling towers, air-conditioning systems, hot water systems, and decorative fountains. The incubation period typically ranges from 2 to 10 days. Clinical manifestations include fever, cough, dyspnea, headache, gastrointestinal symptoms, and confusion, with severe cases leading to respiratory failure, septic shock, or multiorgan dysfunction. Diagnosis is primarily based on urinary antigen testing, culture, and polymerase chain reaction (PCR). Prompt treatment with macrolides or fluoroquinolones significantly reduces morbidity and mortality. Preventive strategies focus on proper maintenance and disinfection of water systems to control bacterial growth. Early diagnosis, appropriate antibiotic therapy, and effective public health measures are essential for reducing disease burden and preventing outbreaks.