Small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids and contain an abnormal isoform of a cellular protein which is a major and necessary component. The abnormal (scrapie) isoform is PrPSc; the cellular isoform is PrPC. The primary amino acid sequence of the two isoforms is identical. Among diseases caused by prions are scrapie, bovine spongiform encephalopathy, and Creuzfeldt-Jakob Syndrome.