"Basicly they just use the horse's bloodstream to develope the anti-bodies or serums needed by injecting small amounts of the disease or venom. "
And lots of folk are allergic to horses. So one of the reasons you can't buy anti-venom for your first aid kit (besides cost and need for refrigeration) is that many people will respond to the anti-venom by going into anaphyllactic shock. So the hospital routine is shot of anti-venom, shot of epinephrine, when patient starts to recover, repeat, repeat, repeat.