1974 outbreak of Legionnaires' Disease diagnosed in 1977. Clinical and epidemiological features.

An illness characterized by high fever and pneumonia struck 2.9% of a group of the Independent Order of Odd Fellows attending a convention held in Philadelphia, in September, 1974. The convention headquarters was the hotel where the American Legion met in July, 1976. The epidemiological, clinical, and laboratory characteristics of the illness were nearly identical to those of the disease in American legionnaires in 1976. Illness was significantly associated with attendance at one convention activity held on Monday morning, Sept. 16, 1974, in the grand ballroom of the hotel. A serological survey in February and March, 1977, showed that people who had attended the convention and became ill were more likely to have raised indirect fluorescent antibody titres than persons who had attended and remained well. The illness seen in Odd Fellows members in September, 1974, was caused by the legionnaire's disease organism.