Disease, variant Creutzfeldt-Jakob: Abbreviated vCJD. A human disease thought due to the same infectious agent as bovine spongiform encephalopathy (BSE), or mad cow disease. Both the human and bovine disorders are invariably fatal brain diseases with unusually long incubation periods measured in years, and are caused by an unconventional transmissible agent, a prion, resulting in the deposition of amyloid tissue that causes a breakdown of brain tissue leaving the infected brain with a "spongy" ("spongiform") appearance. The disease in humans is sometimes called new variant Creutzfeldt-Jakob disease (nvCJD).

The BSE agent has been identified in the brain, spinal cord, retina, dorsal root ganglia (nervous tissue located near the backbone), and the bone marrow of cattle experimentally infected with this agent by the oral route. In addition to cattle, sheep are susceptible to experimental infection with the BSE agent by the oral route. Thus, in countries where flocks of sheep and goats may have been exposed to the BSE agent through contaminated feed, there exists a theoretical risk that these animals may have developed infections caused by the BSE agent and that these infections have been maintained in the flocks, even in the absence of continued exposure to contaminated feed (for example, through maternal transmission).

Everyone with vCJD appear to have eaten beef and beef products at some time during their lives (but then so have most of the population). Investigation of a cluster of cases with vCJD in the UK disclosed that most of the people were likely infected through their diet. Beef carcass meat appears to have been contaminated with the BSE agent in butchers' shops where cattle heads were split.