A protein particle that is the agent of infection in a variety of neurodegenerative diseases, including bovine spongiform encephalopathy, Creutzfeldt-Jakob disease, and scrapie. Prions are the only known infectious agents that do not contain DNA or RNA. They derive from a normal body protein that becomes irreversibly misfolded, and they proliferate in the body, possibly by acting as a template for further protein misfolding.