You're right -- in order for you to be exposed to HIV or STIs, your sex partner(s) has to be infected. So, if you're in a relationship and you've both tested negative for STIs recently, and neither of you are having sex with anyone else, you're not at risk for getting or giving an STI.  A person can have an STI and spread it to others without knowing it. You can't tell whether or not a person has an STI just by looking at them which is why its important to get tested for STIs, even if you're feeling ok.