St. Louis encephalitis: the Chicago experience.

The largest laboratory-documented outbreak of St. Louis encephalitis (SLE) in the United States occurred in the Chicago metropolitan area in the summer and early fall of 1975. Of 1,456 illnesses investigated, 326 cases of confirmed or probable SLE and 420 cases of suspected SLE were found in the six-county area. The outbreak peaked in early September, but cases continued to occur through October. Cases clustered geographically in the southwestern suburbs. There was a direct correlation between age and attack rate, severity of illness, and mortality rate--all of which increased with successive age groups. Thirty-six persons died. Males and females were equally affected. This epidemic marked the first time that St. Louis encephalitis has been documented in the Chicago area.