On death we separate and usually connect up with those we are like and commune. However profoundly discordant individuals also do the same and create their own hell. some even run free.
All this provides a cast of bad actors quite able to infiltrate a body and spirit for its own pleasures. Thus the whole phenomena of possession has a plausible explanation once we accept the reality of spirit.
The second thing worth noting is that at least one psychologist has concluded that mental illness is best explained by all this. So we are not totally whistling in the dark.
Obviously meddling in spiritual matters opens access or at least draws unwelcome and dangerous attention that can lead to possession. The Ouija board is the best example of this actually happening.
I will go further. All mental illness should be first challenged with an exorcism simply to eliminate such an eventuality. If this proves efficacious, then a vast fortune is saved on mental health care. It is also a cheap way to find out for sure.. .
St. Catherine of Siena Exorcising a Possessed Woman. ( Public Domain )
Assyrian demon Pazuzu, 1st millennium BC. ( CC BY SA 3.0 )
L'Exorcisme (The Exorcism) by André Brouillet. ( CC BY SA 3.0 )
1519 painting of an exorcism. ( Public Domain )