Prions have a distinct extracellular form but the extracellular form is entirely protein. The prion particle does not contain any nucleic acid. However, it is infectious and prions are known to cause a variety of diseases in animals such as scrapie in sheep, bovine spongiform encephalopathy in cattle (BSE or 'mad cow disease'), chronic wasting disease in deer and elk and Kuru and a form of Creutzfeldt-Jakob disease (CJD) in humans. No prion diseases of plants are known.