Prions are devoid of nucleic acids and they are composed mainly or exclusively of protein PrPSC, that is a conformational variant of the normal cellular prion protein PrPC, encoded by a chromosomal gene. Conversion of PrPC into PrPSC is a posttranslational process which is accompanied by the acquisition of high b-sheet content. Human prion diseases may be of sporadic, genetic or infectious origin. Human activity caused a 'mad cow disease' epidemic, iatrogenic Creutzfeldt-Jakob disease (CJD), and lately, a new variant of CJD, which is thought to be a result of transmission of bovine prions to humans. Prion diseases are always fatal, and there is a need to develop effective methods of prevention and therapy for these disorders.