placebo effect goes both ways. A client of mine thought she had yeast infection, and though there is no yeast in her system, she still feels it and treats it.
A guy was told by his doctor that he had incurable cancer. He went home and died. Turned out the assistant mixed up the test results, and our guy had nothing.
These two are the negative placebo effects.
Positive placebo effects work the same: you think you are getting something that will cure you, and you get well.
Because all disease is caused by emotions, the positive emotion of hope and trust bring about improvement in one's condition.
Not all placebo works for most people: you need to have a predisposition for actions consistent with getting better, and not a victim mentality, or a taker mentality. If you have those, placebos won't work.