Legionnaire bacteria discoverers. Portrait of the American bacteriologists Drs. Charles C. Shepard (1914-1985) and Joseph E. McDade (left), who isolated and identified the bacteria which causes Legionnaire's disease. They named the bacteria Legionella pneumophila. The organism 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, shower- heads and air-conditioning systems. The disease, also called legionellosis, often causes fatal pneumonic lung damage in the elderly and unfit. Symptoms can be treated with antibiotics.