Unfortunately, for someone who believes they are the victim of a curse or hex, this can sometimes prove quite real.
In the field of medicine, a ‘placebo’ is an inert substance given to a patient which brings about an improvement in the patient’s condition – because the patient expects that the substance will help, and so their perception of the symptoms changes accordingly. But suppose instead that the patient expects the substance will do them harm. This has also been shown experimentally to occur. A ‘placebo’ that makes the patient worse is called a ‘nocebo’.
The mechanism by which curses or hexes work need not be ‘paranormal’ at all, but completely psychological. It may be enough simply or a person to believe or expect that they will come to some kind of harm in order for this actually to occur.