Syphilis is a highly contagious disease spread primarily by sexual activity, including oral and anal sex. Occasionally, the disease can be passed to another person through prolonged kissing or close bodily contact. Although this disease is spread from sores, the vast majority of those sores go unrecognized. The infected person is often unaware of the disease and unknowingly passes it on to his or her sexual partner.

Pregnant women with the disease can spread it to their baby. This disease, called congenital syphilis, can cause abnormalities or even death to the child. Syphilis cannot be spread by toilet seats, door knobs, swimming pools, hot tubs, bath tubs, shared clothing, or eating utensils.