Modern medicine has only been around for a brief instant in the history of humanity. Penicillin isn't even 100 years old. And yet somehow we've been able to survive and even thrive for the vast majority of our existence. How is that possible?
Our bodies naturally have systems in place to keep us healthy and free from the common modern ailments: heart disease, diabetes, cancer, dementia... The further we stray from our EEA way of living, the less effective these systems are at keeping us healthy. Since we are social creatures, our genes select for hosts that do the best job at being social. I believe that the reason shamans and medicine men (and the placebo effect for that matter) work is because our bodies are positively responding to the validation of a respected member of our social group reaching out to us, connecting with us, and essentially saying: “You are an important and valued member of our social group. We want you to get better so you will stick around and continue to be part of the group.” And that validation triggers all of those defense and repair systems to kick into high gear.