I'm curious what you believe about demons, whether they have physical bodies, or whether they exist as evil influences in the mind. Some hard nosed materialists believe in memes, or ideas that have some of the properties of living entities. But not physical bodies.
I think this is an excellent topic for a whole thread. (Not my beliefs, but what the Church teaches). Perhaps when I have time I will start one.
Briefly, demons are fallen angels and like angels they have no bodies. They are superior in intelligence to humans, but they are not omniscent like God. There is a really good book by Peter Kreeft about angels and demons, but I will look for some information online and post a thread when I get a chance.