What I believe is that the subconscious mind can create all sorts of physical manifestations that would make it appear to look like unseen forces are physically attacking including to the person who is directly experiencing it. This is also what parapsychologists believe in regards to hauntings that involve bites, scratches, etc (and even these cases are extremely rare). Parapsychologists maintain that poltergeist phenomenon including physical symptoms are created in the mind of the experiencer.
In those rare cases, and where natural explanations can be ruled out I too think that the very alive, human brain is key to solving the mystery.
Much like stigmata (when people display the wounds of Christ often as described in books, but not accurate to what these would have actually looked liked) I believe that scratches, bites, and bruises sometimes attributed to nasty, demonic ghosts are actually created by the subconscious of the person experiencing it, and that is exactly why psychology more so than anything else often works in putting a stop to these things.
In my opinion it is parapsychology, and not sensationalised tv shows, and films that have come up with the most reasonable answers to this question to date.
Susan St Clair