Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. Any sexually active person can get syphilis through unprotected vaginal, anal, or oral sex.
Syphilis is spread by direct contact with a syphilitic sore (called a Chancre). Sores can be found on the penis, vagina, anus, rectum, lips, and mouth. Syphilis can also be spread from an infected mother to her baby.