There are physical illnesses that cause symptoms which could be mistaken for mental or "spiritual" illness by the ignorant. People in ye olde bible times didn't know any better fortunately we know more - I hope.
Epilepsy of which there are different forms btw originating from different parts of the brain
Brain tumors
TIA
Brain injury
MS
Parkinsons
Side effects from medication - the list goes on.
Not everything is as black and white as demons or crazy.
JWs are a combination of willfully ignorant and extremely suggestable with varying degrees of hypochondria.