DefinitionSyphilis is a frequently diagnosed sexually transmitted disease.Alternative NamesPrimary syphilisCauses, incidence, and risk factorsSyphilis is a sexually transmitted, infectious disease caused by the spirochete Treponema pallidum. This bacteria causes infection when it gets into broken skin or mucus membranes, usually of the genitals. It is most often transmitted through sexual contact, although it also can be transmitted in other ways.Syphilis occurs worldwide. Syphilis is more common in urban, rather than rural, areas, and the number of cases is rising most rapidly in men who have sex with men. Young adults, ages 15-25, are the highest-risk population. People have no natural resistance to syphilis.Because people may be unaware that they are infected with syphilis, many states