How does exercise/fitness affect the immune system?

Yes, yes, yes.: Trust me, yes. Now you're getting into a lot of the biology and science of the human body, detailed analysis of immunity and the immune system, the plasticity and flexibility of the brain--once thought to be set in its ways, the human mind (the brain is physical matter, the mind is much much deeper, and machine will never even approach the complexity of the mind (no matter what cousin ray says)!