Demonic Possession, also sometimes known as Demon Possession, is the supposed violation of the human body and soul by evil forces, normally labelled as demons under Christian faith - though almost all religions (even those outwith Christianity) have similiar belief in "unclean spirits".
Demonic Possession was once believed to be the main cause of all misery in the world and healers, priests and other holy men and women were tasked with the unenviable job of exorcising the often violent spirits out of the body and mind of the afflicted.
Demonic Possession was likely used to explain very real conditions such as psychosis, hallucinations, multiple-personality-disorders, sleep paralysis and other medical ailments - with exorcism being seen as the only way to cure these conditions in societies that had not yet began to think in scientific terms.
Due to the fact awareness has spread on medical conditions (especially mental health) most mainstream churches are wary of performing exorcisms in the modern age but some will perform the ritual if they believe that evidence falls in favor of an actual possession rather than an illness of mind or body.
Science is normally critical of such claims but a controversial psychologist once spoke of how he at least believed that their was such a thing as "spiritual evil" and that it could account for some behaviors normally attributed to sociopaths - his believes however are not shared by the majority of scientists.
Without science, however, there are many who still believe that evil or restless spirits can take possession of others and they look to their priests, tribal leaders, shamans, or similar figures, to aid as they have done for centuries.
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- In the original theological texts, the symptoms of demonic possession was mental rather than a physical change, it included such aspects as sudden internal rage, mindless anger, physical strength and omniscience. But for some reason in films such as Evil Dead, and The Exorcist, it was taken as a literal physical change, and the possessed could for some reason shapeshift. Even some videogames featured demons that can dramatically warp his/her host's bodies into demonic forms that close to, if not 100% identical with the said demons' own original forms (ex. titular demon of Diablo game series can warp his host's appearance into his own casual reptillian demonic forms both to instill fear to his foes and suit his fighting style). However, none of this is mentioned in The Bible.