Transmissible pathogenic agents that cause a variety of neurodegenerative diseases of humans and animals, including Scrapies in sheep and goats, bovine spongiform encephalopathy in cattle, and Kuru and Creutzfeldt-Jakob disease in humans. They are unlike any other infectious pathogens because they are composed of abnormal conformational isoforms of a normal cellular protein, and referred to as prion protein (PrP). Prions are extremely resistant to inactivation by sterilization processes and disinfecting agents.