If you let syphilis go untreated for several years, it can damage your heart, blood vessels, brain, and nervous system. Syphilis can cause blindness or paralysis. It increases your chances of getting and spreading HIV, the virus that causes AIDS. Over time, it can damage your organs and even lead to death.
And, of course, if you’re not treated, you’ll likely spread the STD to your sexual partner. But it’s easily treated and curable in its early stages.