Infectious proteins are present in
(1) gemini viruses
(4) satellite viruses
(2) 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 of 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.