Well, ok. If you two both have cuts and somehow press your cuts together so that your blood mixes, you can share infections.
Or if you drink from someone's glass or share someone's chapstick, you could get oral herpes.
Most sti's are transmitted through exchange of body fluids. That happens most commonly during sex, but can happen from kissing or certain other circumstances as well, depending on the infection.
Is there something in particular you're worried about?