Horses are always working on their position in the herd. Biting each other is just a more direct form of saying, "Move it, NOW" and your knee just happened to be there. The people who run the place should know their horses well enough to not let someone get too close to another horse. Shouldn't have happened, blame the people, not the horses.
As for rabies, most horses get rabies shots routinely, I can't imagine a public facility not having their horses immunized on a regular basis.
IMO, you need to go be friends with 1 very gentle and sweet horse, it will get you past all the horse fears. The OCD is a whole different issue, therapy is going to have to work on that.