False-colour transmission electron micrograph (TEM) of the Gram-negative bacterium Legionella pneumophila. Each bacterium has a single flagellum which is faintly visible in between the cells. 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: x3,500 at 5x7cm, & x1,725 at 35mm size.