The placebo effect is a simple psychological phenomenon affecting only the human body itself (i.e. not bringing changes to the world outside the body itself, which is literally directly regulated by the nervous system), and requiring a total of zero supernatural things.

It's just the interaction of the nervous system with various organs of the body. Aside from placebo and nocebo, this may also lead to psychosomatic disorder, and long-term stress wear and tear. Certain expectations or stressors influence the way organs are regulated, which may lead to positive or negative outcomes depending on the context.