Prion diseases are all, fortunately, rare. Unless we throw some cannibalism (actual or hi-tec) into the mix - then they can increase dramatically in prevalence. It seems likely that most, if not all, prion diseases occur as rare genetic mutations. I very briefly mentioned Fatal Familial Insomnia in class. This is an extremely rare prion disease caused by an inherited genetic mutation. The mutation responsible has been found in just 50 families worldwide. Although patients frequently do not show symptoms until middle age or later the subsequent progression into complete sleeplessness is untreatable, and ultimately fatal.