Mental Illness vs. Demon Possession

Mental illness is a fairly recent idea (in terms of human history). As far as I know, the concept did not exist at all at the time the Bible was written. I have heard some Christians speculate that some of the instances of demon possession recorded in the Bible may have been cases of schizophrenia or other mental illnesses. For example (many others exist): [Matthew 8:16-17](http://www.biblegateway.com/passage/?search=matthew%208:16-17&version=ASV) : > **16** And when even was come, they brought unto him many possessed with demons: and he cast out the spirits with a word, and healed all that were sick: > > **17** that it might be fulfilled which was spoken through Isaiah the prophet, saying: Himself took our infirmities, and bare our diseases. Is it possible that some/all "demon possessions" in the Bible were simply cases of curing mental illnesses? If the answer is "some," can we know the difference, and if so how?