I really don't know. I've researched some mental illnesses, and the doctor pointed out that she had mental illness, and the symptoms coincided with her "possession" symptoms.
But then you get cases where, supposedly, the victim yells in languages they don't know, are stronger than usual, and develop marks in their skin that just appear.
If all that is true, I don't think it's mental illness.
But unless I see it for myself, I lean towards mental illness.
People with mental illness were treated worse than shit.
I remember this story about this couple that had a mentally retarded child, and they kept her locked up in the attic, and people didn't even know she existed. They kept her locked up because they were ashamed of her.