While we are living on the Earth we are subject to all the passions available to people - food, alcohol, sex, etc. Once we pass on and leave the physical bodies behind, these passions do not instantly disappear. Lesser developed souls who lose their physical bodies are obsessed with reaquiring these passions and will go to any lengths in order to do so - including hijacking another body.
Anybody who is in a weakened form of any kind could be succepatable to these invading souls. A weakened form could be anything from depression to mental instability. When an invading spirit finds a person in this state it merely forces the 'legal' owner of the physical body either completely out or partially out and have majority, if not total, control of the body.
We on the Earth have known about this for a very long time, but not understood it. We have called it 'Multiple Personality Disorder', 'Schizophrenia', 'Split Personality', and many more. Unfortunately, with our medical practitioners not understanding these, they have called them diseases and attempted to treat the problem with drugs and medication. It is possible to remedy the problem with a form of exorcism - but not the kind as depicted by Hollywood.
