[mattmcknight]

I think this largely right, but resistance generally does not just evolve in the animals "that do not need it to recover", as all parasites/bacteria are more likely die in that case, but in more extensive cases where some of the parasites/bacteria survive and there are more reproductive events that can introduce a resistant genetic variation. This is why some consider that people not completing the full course of antibiotics- only taking them until they feel better, may be a greater contributor.