In the olden days, primitive people laughably believed that mental illnesses and epileptic fits were caused by evil demons possessing the bodies of humans. They ridiculously treated these maladies with exorcism, a one-time process in which the demons were expelled, allowing the person to return to normal life.
In these enlightened times of course, there is no "cure" for mental illness, which is now humanely treated with neuron-destroying drugs, involuntary commitment, neverending codependency with psychoanalysts and intermittent lifelong bouts of hospitalization with other raving lunatics. Thank God for progress!
Exorcism is one of the most widespread practices in human history. Almost every known religion has some context in which possession is considered possible, although the interpretation of such possession can vary widely. Various religions, such the ancient Greeks and practitioners of Voudoun (or "voodoo," if you're a neanderthal, non-PC bastard), actually embrace possession as part of their religious rituals. The difference in voudoun, of course, is that the possessing spirit (known as a loa) politely and considerately leaves at the end of the ceremony, with the possessed person's body in pretty much the same shape as he or she left it.
When an invading spirit is not so polite and considerate as a loa, exorcism is called for.