How much chance there is to get std, when having sex with a stranger?

Difficult to say: Unfortunately there is limited data to suggest exactly what the odds are of you contracting an STD when having sex with a stranger. But you can decrease your chances by best practicing safe sex, including condom(male/female) and/or dental dams etc to prevent direct skin to skin, skin to mucous membran or mucous membrane to mucous membrane contact. You can still readily get a STD from a friend ..

STD risk: Huge. It is very risky to have unprotected sex with anyone you are not certain about. Besides HIV, you need to be concerned about herpes, and several others.