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In 1995 a fatal epidemic of spongiform encephalopathy appeared in Great Britain. The new epidemic condition was clinically and pathologically similar to Creutzfeldt-Jakob disease (CJD), a rare sporadic encephalopathy, and was called “variant CJD” (vCJD). The vCJD epidemic was detected by active epidemiologic surveillance, which had been set up in response to a British epizootic of another new disease called “bovine spongiform encephalopathy” (BSE). Widespread cattle exposure to the BSE agent (a prion) had presumably been occurring in Britain since about 1982, when changes in the rendering process allowed infected material to contaminate cattle feed. Concern that the bovine disease might transmit to humans via beef consumption led to the active surveillance program. Tragically, BSE transmission to humans appears to have led to over 100 cases of vCJD. Millions of affected cattle were eventually slaughtered throughout Europe. Cattle are now routinely tested for BSE at 24–30 months of age, and affected herds are destroyed. These measures have exponentially reduced the BSE epizootic. Britain and other European countries that received contaminated beef or cattle feed nevertheless remain concerned about the combination of nearly universal past human exposure in Britain, variable exposure elsewhere, an unknown incubation period, vCJD case totals that continue to mount, and worst-case predictions that extend to many thousands of eventual human cases.