An American woman who levitated, demonstrated paranormal psychic powers and spoke foreign languages unknown to her was clearly demon possessed, according to a board-certified psychiatrist and associate professor of clinical psychiatry at New York Medical College.

Real-life case of demon possession documented

The online article is located here:
New Oxford Review

The article is convincing but i still find it hard to believe.

why is it always a demon?

because they're not happy souls, seeking the downfall of man; if there is no God, than why do they still have possession?

Very interesing article. Not often you hear people saying it can be nothing but.