It is widely thought that demonic possesion must first begin with a haunting. The demon must first enter the home of the possessee
Demons can posess any living creature.
These are the "main" causes of demonic possesion:
Obsession by a demon with a mortal.
Obsession with the ocult.
Drastic behavior change.
Knowledge of exotic languages: i.e.: Latin, Aramaic...
Aversion to the Bible.
Knowledge past one's own personal memory: i.e.: Ancient texts, Legends, People, Events.
Change in voice tone.
I don't even believe in demons, persay. I believe that it can't be disproved, so I say nothing on the subject. What I do say is: Nothing scares me half-as-much as the thought of a possessed person near me. Or, people possessed by an evil not able to be defeated. It creates a lump in my throat. It's the look in a stranger's eye, "almost like they know you... deep down", and the slight curl of the corner of a mouth, showing unearthly evil beneath... demon.