Prion

short for proteinaceous infectious particle (-on) that lacks nucleic acid (by analogy to virion); a type of infectious agent composed only of protein

Description
Prions cause a number of diseases in a variety of animals, including scrapie in sheep, bovine spongiform encephalopathy (BSE, also known as mad cow disease) in cattle and Creutzfeldt-Jakob disease in humans. All prion diseases affect the structure of the brain or other neural tissue, and all are untreatable and fatal.