The idea of people possessed by evil spirits against their will is as old as the hills. Often there is thought to be a special way a person has become possessed. It might be the result of a curse cast on them by a sorcerer, or some unfortunate mishap such as stepping over a dead body. Or the demons can have been invited, in a Satanic ritual. More often, at least in recent Christian tradition, there is no initiating moment that can be identified -...