It's probably because maintaining the immune system at high levels costs more energy. You probably know that antibiotic use is rampant in industrial livestock.

It's probably because maintaining the immune system at high levels costs more energy. You probably know that antibiotic use is rampant in industrial livestock. But do you know precisely why? Antibiotics aren't just given prophylactically to prevent infections; constant low doses actually *increase the animal's size*. The animals can put more energy into growing larger, and spend less on their immune system.