Syphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum subspecies pallidum. The primary route of transmission is through sexual contact; it may also be transmitted from mother to fetus during pregnancy or at birth, resulting in congenital syphilis.reponema pallidum subspecies pallidum is a spiral-shaped, Gram-negative, highly mobile bacterium. Three other human diseases are caused by related Treponema pallidum, including yaws (subspecies pertenue), pinta (subspecies carateum) and bejel (subspecies endemicum).