Legionella bacteria. Coloured scanning electron micrograph (SEM) of Legionella pneumophila, the bacteria known to cause Legionnaires' disease. Legionnaires' disease, also known as Legionellosis, is associated with water-based aerosols often resulting from poorly maintained air conditioning cooling towers and potable water systems. The disease got its name in 1976 when an outbreak of pneumonia occurred at an American Legion convention in Philadelphia.