 Syphilis is transmitted primarily by sexual contact or during pregnancy from a mother to her fetus ; the spirochete is able to pass through intact mucous membranes or compromised skin . It is thus transmissible by kissing near a lesion , as well as oral , vaginal , and anal sex . Approximately 30 to 60 % of those exposed to primary or secondary syphilis will get the disease . Its infectivity is exemplified by the fact that an individual inoculated with only 57 organisms has a 50 % chance of being infected . Most ( 60 % ) of new cases in the United States occur in men who have sex with men . It can be transmitted by blood products . It is tested for in many countries and thus the risk is low . The risk of transmission from sharing needles appears limited . 
