It is a remarkable property of the human body that a person's belief in the effectiveness of a medical treatment can itself influence symptoms. For example, if patients complaining of back pain are given "medicines" known to have no active ingredients--such as colored water or sugar--and told that these are effective drugs, a significant number of them will genuinely improve and report that the treatment was successful. Because of this effect, scientific testing of drugs must establish not only that patients respond to them, but that they respond more to them than they do to a placebo. Such a test is called a placebo-controlled study. Recently, it has even been shown that "mock" surgery can have similar effects, and so some surgical techniques must be studied with placebo controls.
The reverse effect, that patients who believe that they should be ill can exhibit genuine symptoms, is known as sociogenic illness.