Syphilis is a sexually transmitted infection caused by a bacterium (Treponema pallidum). It develops in three stages, each characterized by distinct but not systematic symptoms. When symptoms do occur, they are painless, temporary and often unobtrusive. Initially, the bacterial entry point leaves a small, painless pimple that disappears on its own in a few days. Then, the bacterium may temporarily cause the appearance of more or less large and numerous coppery patches, most often on the palms of the hands or soles of the feet. More rarely, it can cause outbreaks of pimples on the chest or back.

The bacterium is transmitted by direct contact with these various lesions, whatever the sexual practice: fellatio, anal penetration (even with a condom), finger play, anilingus, sex toys, fisting, etc. Even when kissing, if the partner has a syphilis lesion in the mouth. The rules of safer sex can reduce the risk of transmission, without completely eliminating it. Condoms do not cover the entire genital area, and contact between partners' bodies can lead to transmission of the bacterium.

Syphilis can be treated well in the early stages of infection with penicillin. If the infection progresses for longer, organ damage will be irreversible, even if the bacteria are destroyed by treatment. In the event of a diagnosis, it is essential that sexual partners undergo parallel treatment. Otherwise, there's a risk of re-infection despite treatment (ping-pong effect).

There are several ways of detecting syphilis, depending on history, exposure or symptoms. Sexually active people should be screened at least once a year (screening recommendations).