Beginning in the mid 1980s, an epidemic of bovine spongiform encephalopathy (BSE) was seen among cattle in the United Kingdom. The cause was recognized as a prion agent transmitted in the meat and bone meal products used as animal feed, a practice that was banned in 1988. Unfortunately, by that time, infected cattle had already entered the human food supply, and by 1996, several individuals were identified with a variant form of Creutzfeldt-Jakob disease (CJD), caused by the same infectious agent that had caused the disease in cattle. In the United Kingdom, 137 cases of variant CJD have been reported to date. The culprit prion protein causes no immune response and contains no nucleic acid, making preclinical detection impossible. It is also very resistant to conventional forms of inactivation and decontamination. Although no cases of variant CJD have been linked to transfusion of infected blood, there is concern that the prion agent may be transmissible in blood, and evidence from sheep models suggests that it can be. Because it is impossible to detect presymptomatic individuals who carry the etiologic prion agent, the United Kingdom has adopted measures to secure the safety of its blood supply and to minimize potential routes of infection.