Do you believe in demonic possesions?why?

neither do i experienced it nor seen one possessed, i mean in real life, not those i've seen on TV/documentaries. 'God please forbid. i believe it so because the 'Bible tells it so, e.g. Mark 5:15 KJV - And they come to Jesus, and see him that was possessed with the devil, and had the legion, sitting, and clothed, and in his right mind: and they were afraid.

They claim it was "based" on a true story. This means some events have been changed. I thought that was a great movie though I don't know how much I actually believe it. I think in order to believe in something of this nature, I would have to see it with my own eyes. This being said, it's definitely not something I would want to experience or have to see.