Abnormally folded protein that can induce healthy folding proteins in the brain to convert to the abnormal form. The mysterious protein causes many progressive neurodegenerative disorders known as transmissible spongiform encephalopathies. They afflict both humans and animals and include Creutzfeldt-Jakob disease (CJD) in humans and mad cow disease or Bovine spongiform encephalopathy (BSE) in cattle and livestock.