Legionellosis--the 1980 San Francisco outbreak.

During the first half of March 1980, an unseasonal outbreak of legionellosis, which involved at least 14 of approximately 1,000 workers, occurred in a 6-month-old office building in San Francisco. An epidemiologic investigation revealed that persons 50 yr of age and older were at increased risk of disease. The outbreak was temporally associated with operation of the building's air conditioning system. In addition, Legionella pneumophila Serogroups 1, 4, and 6 were identified in water samples taken from a rooftop air conditioning cooling tower located directly across the street from an area of recent excavation.