As you didn't know you were infected, you can't hold yourself responsible for infecting anyone else. The only person at fault is the girl that gave it to you, assuming she knew. (If not, then it's the person that gave it to her, and so on up the chain.)
You also aren't responsible for not using protection after you were infected. You didn't know. (In hindsight, protection probably would have been a good idea, but you can't do anything about it now, so forget about it.)
In any case, the ethical thing to do would be to inform them, if you can. You don't have to include your name or anything, like the poster two posts above this says. Just give a rough date, so they can inform anyone they need to also.
