The best way to learn is to experience. Our body's self-defense works just like that.
Our bodies get sick, and heal, and come out a little bit stronger every time.
Animals work just like that.
Giving them time to explore and experience, makes their bodies heal from diseases and grow resistant to it for the next time.
This is being stored in the white bloodcells, allowing their body to recognise diseases and act accordingly, long before they get infected.
The body's self-defense system evolves all the time, and works best in a natural environment, like it has for thousands of years.
Animals with a natural defense system have way fewer cases of salmonella and other diseases.