Demons Cause Physical Problems, but Why?

Even in the first century before modern medicine, they didn't automatically assume that such a person had a demon. So demons can cause physical prob s, but merely having a physical problem is not enough to diagnose a demon. And what about mental problems? Do we see people on the bible displaying mental abnormalities because of demons? We do. The classic case is the gaderine or garrisene demoniacs that we read about in matthew mark and luke.