n. a contagious disease caused by infection with the spirochete bacterium Treponema pallidum. Syphilis is usually a sexually transmitted disease, but it can be transmitted through a break or cut in the skin or mucous membrane; it can also be transmitted by an infected pregnant woman to an unborn child. Untreated, syphilis progressively destroys body tissues, particularly those of the heart and nervous system. See also cerebral syphilis; general paresis.