Legionnaires' disease, also known as legionellosis, is a severe form of pneumonia caused by the bacterium Legionella pneumophila. The disease is named after an outbreak that occurred in 1976 during the American Legion convention in Philadelphia. Other synonyms for Legionnaires' disease include Pontiac fever, which is a milder version of the disease, and Legion fever. Legionellosis can present a wide range of symptoms such as fever, headache, muscle pain, and difficulty breathing. It is primarily contracted through inhaling contaminated water droplets from hot tubs, showers, or air conditioning systems. Proper maintenance and disinfection of water systems can help prevent the spread of Legionnaires' disease.