Syphilis is caused by an infectious bacteria. The bacteria enters the body during close body contact through small breaks in the skin.

During sex syphilis can be passed on through any close body contact. Syphilis isn’t just passed on through penetrative sex. It can also be transmitted through skin-to-skin contact including kissing, fingering, mutual masturbation, rimming and fisting.
Using condoms during penetrative sex will help to reduce the risk of picking up or passing on syphilis.

If you’ve been diagnosed with syphilis and have received treatment, you can still pass syphilis on for at least 2 weeks after completion of that treatment. Avoiding sex for 2 weeks after treatment will prevent you from passing it on.

If you’re having lots of sex, regular syphilis testing means you can be treated for syphilis quickly if you have it. Getting treated quickly reduces the chance of passing it on to other people.
To find out more about syphilis prevention during pregnancy see here