Syphilis starts when the bacteria Treponema pallidum enters the body through mucous membranes, usually during sexual contact. The bacteria commonly enter through the genitals, rectum, or mouth, and then enter the blood stream and spreads through the body. In the first stage, syphilis causes a sore called a chancre on the area of the body where the bacteria first entered. In the second stage, the bacteria may cause a skin rash and a variety of other symptoms, including general aching and fatigue. By the time it reaches the third stage, the disease starts to infect major organs - including the spinal cord, brain, and heart - and can cause very serious complications.