You all know the placebo effect; the near-universal belief that people's minds can make them well when a magical sugar pill masquarading as actual medicine is offered. Turns out that this isn't true.
Some number of people get well with no treatment at all. These were the folks who were considered cured by the placebo effect.
There are two caveats, however; there is a real placebo effect for painkillers, and a smaller placebo effect for subjective symptoms.
But, if you need, say, an antibiotic, not so much.