1976 Philadelphia Legionnaires' disease outbreak States Center for Disease Control and Prevention (CDC) in 2017 only use the term Legionnaires' disease to refer to the serious version with pneumonia. CDC investigators quickly discovered outbreaks of respiratory disease caused by "L. pneumophila" dating back to 1959. An outbreak of what was dubbed Pontiac fever occurred in a health department in Pontiac, Michigan in July 1967. No one died. Although caused by the same bacterium, Pontiac fever is a milder ailment than Legionnaires' disease. Pneumonia is absent in those with Pontiac fever. An outbreak of pneumonia in July–August, 1965, at St. Elizabeths Hospital in Washington, DC, which
1976 Philadelphia Legionnaires' disease outbreak 25 had died. Both the first week and later, there were 149 Legionnaires who became sick and 33 other persons associated with the hotel or in the area who also became sick. Of these total of 182 cases, 29 persons died. The U.S. Centers for Disease Control and Prevention mounted an unprecedented investigation and, by September, the focus had shifted from outside causes, such as a disease carrier, to the hotel environment itself. In January 1977, the "Legionella" bacterium was finally identified and isolated and was found to be breeding in the cooling tower of the hotel’s air conditioning system,