A contagious bacterial infection caused by the spirochete Treponema pallidum. It is a sexually transmitted disorder, although it can also be transmitted from the mother to the fetus in utero. Typically, it is initially manifested with a single sore which heals without treatment. If the infection is left untreated, the initial stage is followed by skin rash and mucous membrane lesions. A late stage follows, which is characterized by damage of the internal organs, including the nervous system.