A commonly known psychological idea is the placebo effect, where there is an improvement results from the mere expectation of an improvement, but what happens when there is an expectation of harm? That's where the nocebo effect comes in; if you expect the worst, you get the worst. If someone goes into a test completely believing they're going to fail it, chances are they will because of the nocebo effect.
This image shows another way the nocebo effect can work, if someone sees all the side effects of a medication they're taking, they will start to feel those effects taking place even if they don't actually have them. The nocebo effect is important because it helps to explain why things like voodoo work. In 2002, a study by Ried had people who were allergic to roses were presented with fake ones and they started sneezing.
The nocebo effect is a very interesting concept, and I would love to know more about why it happens. Heuristics may be involved in why the human brain does this, but I want to know more about what's going on in the brain to make both the placebo and nocebo effects happen.