Legionella bacteria. Coloured transmission elec- tron micrograph (TEM) of a group of Legionella pneumophila bacteria, cause of Legionnaires' disease. The bacteria are seen sectioned. They are rod-shaped Gram-negative bacilli. L. pneumophila was first identified as an agent of disease after a mysterious outbreak of pneumonia caused 29 deaths at an American Legion convention in 1976. The bacteria was found living in water- tanks, showerheads and air-conditioning systems. The disease, also called legionellosis, causes fatal pneumonic lung damage in the elderly and unfit. Magnification: x5,000 at 6x7cm size.