Syphilis is a venereal disease spread during sexual intercourse. It can also be passed from mother to child during pregnancy. It is caused by a corkscrew-shaped bacterium called a spirochete, Treponema pallidum. This microscopic organism resides in many organs of the body but causes sores or ulcers (called chancres) to appear on the skin of the penis, vagina, mouth, and occasionally in the rectum, or on the tongue, lips, or breast. During sex the bacteria leave the sores of one person and enter the moist membranes of their partner's penis. vagina, mouth, or rectum.