False-colour transmission electron micrograph of an isolated Gram-negative bacterium Legionella pneumophila. showing its long single flagellum. The bacteria was identified as an agent of disease in 1976 after a mysterious outbreak of pneumonia and other respiratory illness at an American Legion convention in the USA. It was found living in water-tanks, shower-heads and in the cooling towers of air-conditioning systems. The bacteria was subsequently named L. pneumophila and the disease, a form of pneumonia, 'legionnaire's disease' or legionellosis. Magnification: x5140 at 35mm size.