Legionnaires' disease in a prepaid medical-care group in Seattle 1963--75

The community incidence of legionnaires' disease (L.D.) was estimated by a retrospective study of stored paired sera from 500 patients treated for pneumonia in Seattle in 1963--75. The patients were all members of a prepaid medical-care group; 84% were treated as outpatients. Only 5 (1%) had a fourfold rise in antibody titre to the L.D. antigen. Seroconversion to influenza A virus also occurred in 3 of the 5 cases. The course of illness in 4 of the 5 patients with confirmed infection was milder than that previously reported with legionnaires' disease. In addition, 1 child had a fourfold fall in titre, and 23 patients had titres of greater than or equal to 256 without significant change in titre. Only 8 of the 23 had their first serum collected later than 2 weeks after onset of illness, which suggests that the majority had acquired their antibodies before their attack of pneumonia. Prevalence of antibody in the 500 patients was not affected by age, sex, or year in which illness occurred. The incidence of legionnaires' disease based on 5 patients with rises in titre was estimated to be 0.4--2.8/10 000 persons/year.