Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. It may also be transmitted mother-to-child (congenital syphilis).

Humans are the only reservoir and, apart from congenital cases, the only epidemiologically relevant mode of transmission is by direct contact with treponema-rich, open lesions and contaminated secretions from a patient.

After an incubation period of 10 to 90 days (three weeks on average) clinical symptoms appear: at first a primary lesion at the site of infection (chancre), then a series of eruptions on mucous membranes and skin (secondary syphilis), followed by long periods of latency (latent or tertiary syphilis). If untreated, many years after the initial infection, tertiary syphilis lesions might finally appear (visceral, multi-organ involvement, including serious vascular and neurological damage).

Mother-to-child transmission might result in foetal death, perinatal death or congenital syphilis. The latter can be without symptoms or present stigmata or determine multi-organ pathology.

With the widespread use of penicillin, syphilis prevalence had significantly declined after World War II. However, in several industrialised countries a considerable resurgence occurred in the late 1980s.