Exorcism is obviously the attempt to compel non-physical entities allegedly inhabiting a person’s body to depart. These entities are usually identified as demons or even the devil, though these are not the only options and ghosts can also be blamed. Exorcism is also mainly associated with the Catholic church but many cultures have a form of this practice, even the Polynesian ancestors of our very own Maori. Putting aside once again how an immaterial being can interact with and so influence or control a corporeal body, since basically by definition they cannot affect each other, what evidence is cited to support the thesis of possession?
The behaviour of the possessed is often looked at to determine possible possession, personality changes, loss or gain in weight, violent behaviour, nightmares and self-mutilation have all be suggested as signs of demonic influence. To their credit most modern believers in possession admit that these signs can also be explained by mental illness and that the diagnosis of possession should not be rushed into. Levitation and psychokinesis can give more reliable evidence apparently but I’m not aware of any independantly verified instances of this. Besides it could just be a ghost, right?
Unfortunately such medieval concepts of the workings of the human mind and condition do still persist. The harm that this particular belief can do is almost self evident but there are several examples listed below if your imagination has run dry. Ultimately if you become convinced that an entity of pure evil has entered and is controlling your loved one then you may be motivated to do almost anything to rectify that situation, I know I would. You may even come to the conclusion that if harming or killing said loved one is the way to achieve this end then it is better than the alternative, and after all, it’s for their own good.