I had a minor bite once from a fairly large dog - I think it was a Doberman. It didn't cause that much damage but it was extremely painful and I needed a course of antibiotics and about 8 shots to stave off infection. Never again.
If an un-leashed dog is threatening your health/safety, you can and should defend yourself. For me, the potential fault of the runner hinges on whether the dog was, in fact dangerous.
That being said, anyone who lets their excitable animal off the leash in public is a jackass and a hazard to society. They shouldn't be allowed to own animals.