Mental illnesses usually can be traced to the individual's personal negative history like stress, anxiety or emotional trauma. Or perhaps by a defect in the individual's brain or body.

How then can you explain the rapid change of behavior of the individual when clearly there is no evidence of any unnaturalness of the individual's psychological or physical history before the person got "mentally ill"?

But then again, I personally don't know anyone who has been "possessed" before, so... I wouldn't know if anyone actually fit the above description.

I'm just saying, there's always a 50/50 chance of a "crazy" person being mentally ill, or possessed.