Prions are composed of abnormal forms of a host protein termed prion protein (PrP).These agents cause transmis-sible spongiform encephalopathies, including kuru (associ-ated with human cannibalism), hereditary or sporadic Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE) (better known as mad cow disease), and variant Creutzfeldt-Jakob disease (vCJD) (probably transmitted to humans through consumption of meat from BSE-infected cattle).