Any breed can be aggressive
Any breed can be aggressive. I could go on about this forever, but I will try not to. I have to say first off, that in my experience, you are more likely to get bitten by a little dog(yorkies, chihuahua, etc.) It all has to do with the owners, not the breed.
I have met a lot of pit bulls, none of which have ever been aggressive. The difference (and why people have the fear of pit bulls) is the severity of the bite. A chihuahua will bite, and may even take a little chunk of flesh, if it gets a good bite. A pit bull that bites, can take a finger, an, an ear, etc.
Most of the stories you hear in the news about pit bull attacks, never tell you what the human was doing to the dog. Any breed of dog, if not treated, trained or socialized properly will bite. I have a pit bull, like most of the people reading this I assume, and she has never bitten anyone. My 1 year old daughter climbs all over her, pulls her ears, and tail. She pokes at her, and smacks her, and yes, I am trying to teach her to be nice to the dog, but the only reaction she ever gets is kisses. That being said, my dog is VERY protective of my children, though short of someone hurting them, I don't believe she would bite.
She has been attacked by little dogs, and just sat there and looked at them. My biggest pet peeve, is people who own little dogs who don't feel the need to train them like any other dog. Or the people who think it's funny or ok, that their little dog gets aggressive with my pit bull.