Causes
In the vast majority of cases, syphilis is transmitted by sexual contact. It can be transmitted by all forms of sexual contact, including oral and anal sex, and potentially even by kissing.
Syphilis produces a painless ulcer on the body part that's come into contact with an infected person. That sore slowly leaks a clear liquid, which contains many syphilis bacteria. If it touches someone else's broken skin or a mucous membrane (such as the inside of the vagina), it's likely to create a new sore - passing on the infection. These initial sores cause no pain and are often located in hidden areas, so people can transmit syphilis without knowing they have it.
A few cases of syphilis are transmitted from infected mothers to their newborns, but with routine screening this form of transmission has almost vanished.