If you have sex with someone with an STD, will you automatically?
No, STDs are not 100% contagious. In fact, many of them are far less contagious than people think they are. If you have sex with someone who has an STD, the most likely outcome for any particular act of sex is that you will not get it. If you are unlucky, you might get it the first time you have sex with them. But if you are lucky, you could have sex with them dozens of times and not get their STD. It is all a matter of random chance.