A kind of infectious entity made up of only protein which is about 100 times smaller than virus and causes disease in the host body is called prion. They are different from viruses. It is Kuru and Creutzfeldt disease of human central nervous system; Responsible for scrapie disease and mad cow disease in sheep and goats. Stanley Prusiner first observed this disease in 1982. He was awarded the Nobel Prize in 1997 for the discovery of this bacterium.