Take these two cases.
Rabbits in Australia. They’re an introduced species here, and they run amok. A number of diseases have been introduced to the rabbit population to try to cut them down. It works on some of the rabbits, but the ones that recover from the disease then produce stronger offspring. So the farmers introduce another disease, and on and on it goes.
The second case is horse flu in Australia. Horses were getting sick & dying from this horse flu. All the tracks (in South Australia, at least) were closed for a time not long ago because they didn’t want it to spread. Then suddenly there’s a vaccine available for it, just in time for the Melbourne Cup (which took place yesterday).
So here’s my point. Maybe, in order to have a better chance at getting a winning horse, breeders should be letting their animals get sick, get over it, and move on to breed stronger horses, rather than jockeys thinking they have to starve themselves before a race.