If you want to pull humans into it and the "Well if YOU were the HORSE..." That is silly. Pookie precious is not a human. I am not a horse.
As a human....
If scientists found that something my body produced through pain cured some serious life threat, and through my pain a handful, or a hundred, or a thousand lives could be saved . . . I would volunteer . . . Well, I don't think I would volunteer, per se, because I would want something out of it besides personal gratification at helping these others, but nevertheless, I would do it. There would likely be a whole heck of a lot of swearing and death threats, but I would do it.
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We put our horses through much pain regardless. Horses can have bad reactions to vaccinations or wormings. Totally human inflicted. Horses could pull muscles, cramp up, or sprain things in every day work. There are lots of ways we cause pain in a horse in their daily lives.
And in the process of giving birth, the female must undergo severe pain. Yes this happens naturally(as do snakebites), but if you own a horse, your horse was created by causing great pain to another horse. If the breeding that produced your horse was a planned breeding, then it was essentially caused by man.
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The horse is given such small doses, similar to the weakened pathogens of a vaccine, so that the body can effectively defeat it.
And yes, I do find this fascinating and very cool. The body's defense systems, both horse and human, is just so cool to me.