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Periodic increases in the incidence of syphilis are often attributed to social and behavioral changes in the populations affected and trigger public health measures aimed at stopping the outbreaks. To reevaluate this idea, investigators analyzed CDC data on syphilis and gonorrhea for 68 U.S. cities from 1941 through 2002.
The researchers noted a periodicity of 8 to 11 years in the peak incidence of syphilis in most cities. Analysis of gonorrhea cases showed no evidence of such periodicity. To examine the difference between the epidemiologic patterns of these two sexually transmitted diseases, the investigators tested the effect of partial immunity conferred by previous syphilis (but not gonorrhea) infection. Mathematical modeling revealed th…