While it’s not the most common STI in teens, it can still cause problems (especially depending on where you live—it’s more common in different parts of the world). You are diagnosed with syphilis with a blood test. The good news is syphilis is treatable with an antibiotic, but you want to detect it and treat it ASAP.

The first sign of syphilis is an ulcer in your genital region. It doesn’t hurt and happens weeks after you actually catch syphilis. Again, it’s best to get tested and treated as soon as possible to prevent it from spreading to other parts of your body (like your brain, heart and eyes).