Syphilis is a sexually transmitted disease. It is usually transmitted through sexual intercourse. Like HIV, if you get the virus it does not immediately manifest itself. You just don't know you had it until months later sores appear around your body. The only way to verify if you have the virus is through blood screening or get a tissue tested.
Aside from syphilis, there is also gonorreah, hepatitis, HIV and more. That is why people are advised to practice safe sex! Use condoms often!