Exorcism

Exorcism is an ancient practice involving the expulsion of demons from a possessed person's body. The person performing the ritual is referred to as an Exorcist, and is usually a high ranking member of his/her church. While exorcisms are more commonly associated with the Catholic church, exorcisms are performed in almost every religion that involves the belief in evil spirits.

Scientific Views
Many doctors believe that a person who is labeled possessed may actually suffer from a particular mental illness. For those who are successfully exorcised, science attributes their cleansing to the power of suggestion and/or the placebo effect. That is; their beliefs are so strong that the ritual will actually trigger a remission in whatever mental illness they suffer.

Criminal Minds
In the episode "Demonology", Father Paul Silvano killed his victims during exorcisms with a drug known to cause cardiac arrest. He attributed their deaths to the fatal effects of possession.

Sources/Further Reading

 * Gabriele Amorth, An Exorcist Tells His Story. San Francisco: Ignatius Press, 1999. Vatican's chief exorcist tells about Roman Catholic practice of exorcism with numerous anecdotes from his own experience.