The organism that causes syphilis is Treponema pallidum. Syphilis is a sexually transmitted disease that is caused by this bacterium. It spreads from person to person through direct contact with a syphilis sore during sexual activity. The bacteria can enter the body through tiny cuts or breaks in the skin or mucous membranes. If left untreated, it can cause serious health problems, including damage to the heart and brain. It is important to seek medical attention if you think you may have syphilis.