Whether the placebo effect actually works to bolster the immune system or not is debatable. But for the sake of argument let’s assume it does. The theory runs as follows:
Prolonged stress in mammals is a bad thing. Stress is a prolongation of the flight or fight response. Under conditions of stress your body releases all sorts of hormones that divert blood away from the vital organs towards the muscle, slow digestion, increase blood pressure, prevent rest and inhibit the immune system. Those reposnses are all very sensible reactions if you are about to be burned to death by a forest fire since they tend to take you out of danger faster. They are terrible reactions when they are prolonged over a period of weeks rather than days or seconds.
The trouble is that humans are just mammals. We evolved from a stock which couldn’t anticipate the future more than a few days ahead and that live din an environment where something was either immediately fatal or it went away after a few days. For other mammals in the wild stress is a non issue because it only lasts a few days and then vanishes.
Humans unfortunately can anticipate the future and when we think we are likely to die or be permanantly maimed we experience exactly the same reactions that a deer might feel at the approach of a forest fire. That is bad because unlike a fire the stress of chronic disease is prolonged.
If you believe that you won’t die or be permananently maimed then you naturally stress less. That means you sleep better, you eat better, your blood pressure is lower and your immune system isn’t compromised. It shouldnt; suproise anyone that those things will assist in reovery form illness and injury.
Basically the bizarre behaviour isn’t bizarre for animals with limited self awareness faced with simple problems that can be run away from. Humans are self aware, our problems are complex and we ofteh can’t run away from them. IOW we have both socially and mentally out-evolved our own endocrine system. Given a few hundred thousand years the two systems will doubtless run in synch again, but at the moment we are at the point where one system has evolved faster than the other.