A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient.

Sometimes patients given a placebo treatment will have a perceived or actual improvement in a medical condition, a phenomenon commonly called the placebo effect. …

BBC – The contagious thought that could kill you

In the opposite effect, a patient who disbelieves in a treatment may experience a worsening of symptoms. This effect, now called by analogy nocebo (Latin nocebo = “I shall harm”) can be measured in the same way as the placebo effect …