The horse has played a little known but very important role in the field of medicine. Horses were injected with toxins of diseases until their blood built up immunities. Then a serum was made from their blood. Serums to fight with diphtheria and tetanus were developed this way.
It can be inferred from the passage, that horses were
- given immunity to diseases
- generally quite immune to diseases
- given medicines to fight toxins
- given diphtheria and tetanus serums