Legionella bacteria. Coloured scanning electron micrograph (SEM) of Legionella pneumophila bacteria (yellow), the cause of Legionnaires' disease. This rod-shaped, Gram-negative bacillus has a single motile flagellum, which enables it to move. L. pneumophila was first identified as an agent of disease after a mysterious outbreak of pneumonia killed 29 American Legion members at a convention in 1976. It was found living in water tanks, showerheads and air-conditioning systems. The disease is contracted by inhaling water droplets containing the bacteria. It causes fatal pneumonic lung disease in the elderly and unfit. Magnification: x2850 when printed 10cm wide.