The nocebo effect, often shadowed by the popularized placebo effect, is a strange phenomenon in which a person receives negative side effects from expecting negativity.
As stated in the Lilienfeld text "Psychology: From Inquiry to Understanding", the nocebo effect is considered the "evil twin" of the placebo effect. While the definition of the placebo effect "is improvement resulting from the mere expectation of improvement", the nocebo effect "is harm resulting from the mere expectation of harm". [In Latin, placebo means "I shall please," and nocebo, "I shall harm"(harvardmagazine.com)].
As listed in "Psychology: From Inquiry to understanding", one study showed that people who were allergic to roses sneezed in the presence of fake roses. The text also points out that the nocebo effect is common in people who believe in voodoo, blaming the aches and pains they experience on their belief that pins are being pierced into dolls made to symbolize them.
In my own experiences, I definitely believe the nocebo effect is a valid occurrence. Just the thought of getting a headache are enough to give me one and my joints begin to ache just as I think of running marathons. Back in the day when I would pretend to be sick in order to skip school I would commonly fall ill later in the day with the symptoms I had falsely claimed I had been experiencing that morning.