An infectious disease caused by the bacterium Treponema pallidum, which is typically transmitted through direct contact with a syphilis sore, usually during vaginal or oral sex. Syphilis can also be transmitted from an infected mother to her child during pregnancy. Syphilis sores occur mainly on the genitals, anus, and rectum, but also on the lips and mouth. Genital sores (chancres) caused by syphilis increase the risk of sexual transmission of HIV.