Isn't the brain a neat little thing? It can be affected by so many things, big and small, and all it takes is the power of belief.

Yes actually. What you're thinking of is called the nocebo effect. Distrust and other similar things can cause symptoms to worsen and cause problems health wise.

Infact Voodoo is one of the biggest causes of the nocebo effect, which can go so far as to cause death. All with just the power of belief.