Dogs aren't poorly trained, they're let down by their people.

I've been bitten by:
  • a Pekinese - owned by the neighbor of my grandmother, dang dog bit me after I was invited in the house by her owner

  • a German Shepherd - guard dog in the public show barns at a horse show where I was working as an EMT, the dog had already bitten my patient rather severely when she went to put up her tack. She knew the dog and the dog knew her, where I was a stranger to the dog.

Both acts were totally unprovoked on my part other than apparently occupying the wrong space.

As a kid I owned a French Poodle (from Paris no less , his arrival with us back to the States is another story), that bit a neighbor boy. Said child had it coming, even the Animal Control people said so. But the law was the law, and we had to keep our dog chained up inside his fenced yard for 2 weeks. The boy was jabbing our run-away dog with sticks and pine cones, while holding on to his collar. Adam had enough and bit him.

As for Pit Bulls in shelters.... They're popular among urbanites because bull dogs "look tough", and unfortunately the have been bred to fight. I think it's been said that ounce for ounce Chihuahuas are pretty vicious. If they were as big as Pits, shelters would probably be full of them too.