Mad Cow Disease is one of a series of prion diseases - fatal neurological diseases caused by an [abnormal confirmation] of the prion protein. Prion diseases have been known to occur for about 300 years, first occurring in sheep in a condition called scrapie, although the cause was unknown at the time. Prion diseases have now been described in a number of animals, including cows, cats, deer, elk, and others. In humans, it causes Creutzfeldt-Jacob disease (CJD), which occurs spontaneously in 1 in 1 million persons but can ... be passed on genetically.