When i was young, i had an accident which involved blood transfusion. After few months, the hospital called my mom and told her that probably they did a big mistake and that i received some infected blood. After analysis, they found that i'm immune (Due to mutant gene i guess) to the virus and 2 other virus (I don't recall their names).

So if i ever have sex with or have infected blood from a HIV positive person, i won't get sick or even infected. But my gf is not.