I have seen nothing that would lead me to say it was real, however I am still open to the chance that it is. What I belive is a huge factor behind any movement that claims to heal/harm is the individuals power of belief. What you tell yourself has an odd way of happening. If you belive that you will get better there is a better chance (i'm not saying that the belief will cure you, just stating that it helps) that you will get better. The belief that something bad will happen to you will most likely cause something bad to happen (less out of any mystical sense then the fact that you will be paying attention to all the wrong things).
I think things such as curses just creates a time when everything that happens badly can be blamed on the curse.
Other then morphing (different thread) could you give me an example from your personal experience that would lead me to belive? (I'll take your word that it happened though whether I belive it was an effect of the witchcraft I will decide after the example has been said )