What is the definition of a demon, and demon possession?
The Bible gives us the definition of a demon as a fallen angelic creature who has followed satan into a rebellious course as an abuse of free will. Before the flood of Noah's day, these angels could materialize and take on human form, but because of the rampant wickedness caused by them and their freakish offspring, God destroyed the world of that time and made sure that those rebel angels could no longer materialize.
Robbed of that ability, these wicked spirits needed a physical anchor to continue in their agenda to mislead mankind, so they can possess either animate or inanimate objects....people or material things. As Jesus revealed, many can invade the body of just one man.
The skeptics can ridicule such suggestions, but it doesn't make them untrue. If Jesus had encounters with wicked spirits and he identified them, then I will take his word over the skeptic's.
Demon possession is not mental illness, but a weakened mind can make a person more susceptible to their influence. Alcohol too can be an invitation to demonic influence, as many alcoholics are merely fighting an out of control mind. Once a person loses control of their thinking and actions, it's an open invitation.
In countries where spiritism is a cultural practice, demon possession can take many forms, resulting in odd manifestations and extremely strange behaviors. A climate of fear dominates.
This is how I understand demon possession.
I have personally had uninvited encounters with the demons, so I know they exist, and I know how to deal with them, as advised in the scriptures. Every culture has its evil spirits. They are not just figments of a vivid imagination.
You have to know your enemy to fight him.....if you don't believe he exists then you disarm yourself.