Legionnaires' disease, conceptual computer illustration. Legionella pneumophila bacteria are the cause of Legionnaires' disease. These bacilli (rod-shaped bacteria) are Gram-negative. L. pneumophila was identified as a pathogen after a mysterious outbreak of pneumonia caused 29 deaths at an American Legion convention in 1976. This bacterium was found living in water tanks, showerheads and air-conditioning systems. The disease causes fatal pneumonic lung damage in the elderly and unfit.