"Illusion" is a now that points to something that deceives by producing a false or misleading impression of reality. Most people would never consider medical interventions as illusory. But the reality is it can be. There's even a term for it, therapeutic illusion. At its simplest, therapeutic illusion is the false belief of the beneficial effects of medical therapy, even when the interventions might have been unnecessary.
The therapeutic illusion was brought to the fore in the 70s by a general practitioner. He tracked 200 patients who had general symptoms, but their diagnostic evaluation did not yield any specific diagnosis.