1976 philadelphia legionnaires' disease outbreak 1976 philadelphia legionnaires' disease outbreak the 1976 legionnaires disease outbreak, occurring in the late summer in philadelphia, pennsylvania, united states was the first occasion in which a cluster of a particular type of pneumonia cases were determined to be caused by the legionella pneumophila bacteria. the bellevue-stratford hotel, site of the first known outbreak of legionnaires' disease. the hotel closed in november 1976, four months after the outbreak. previous outbreaks were retroactively diagnosed as being most probably caused by legionella bacteria. background