At risk of STD?

I’m a female in high school and I’m seeing somebody, let’s call him Mark. I want to have sex with Mark but I don’t know if he has an std or not. The last person he’s had sex with, Natalie, claims she didn’t have anything to give to him, but she’s had multiple sex partners in the past and she even has a kid. Everyone says she’s infected, so if that’s the case, Mark is potentially infected. Someone give me advice