Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Syphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum subspecies pallidum. The primary route of transmission is through sexual contact; however, it may also be transmitted from mother to fetus during pregnancy or at birth. The signs and symptoms of syphilis vary depending in which of the four stages it presents. Learn more about syphilis, its causes, symptoms and treatment in the following pages.