Treponema pallidum, the causative agent of syphilis, belongs to the family Spirochaetaceae and is a unicellular, spiral organism that cannot be cultivated in vitro.

In contrast with gonorrhea, the number of cases of syphilis reported to the Centers for Disease Control and Prevention(CDC) rose from 30.5 per 100,000 population in 1980 to 54.5 in 1990. Fortunately, the number of reported cases since 1990 has declined steadily,with only 26.2 cases per 100,000 having been reported in 1995. The number of cases of congenital syphilis mirrors this pattern, with 277 cases reported to the CDC in 1980, increasing to 3,865 cases in 1990, and decreasing to 1,548 cases in 1995.

The increased incidence of syphilis in the 1980s may have been associated with the increase in prevalence of human immunodeficiency virus (HIV)infection in the United States. Up to 15%of adolescents and adults who have syphilis are coinfected with HIV.

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