A rare neuro-degenerative disease, a contagious form of spongiform encephalitis, thought to be caused by a slow virus which is usually fatal and has no cure. The main symptom is progressive dementia (deterioration of brain function), and other symptoms which can also occur include wasting away of the muscles, tremors, and a variety of different neurological conditions. Recent research has linked this disease with bovine spongiform enchephalitis (BSE), more popularly known as "Mad Cow Disease," which is thought to be caused by a prion (an abnormally-configured protein).