Prion-related diseases include scrapie in sheep, bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in humans. The infectious agent for these diseases surprisingly contains no nucleic acid, but is a protein(PrP) which exists in two conformations, PrPC and PrPSc. The infectious PrPSc form has a higher β-sheet and lower α-helix content than PrPC. The structures of PrP and models for how PrPSc is able to replicate by converting PrPC to PrPSc are discussed.