http://www.cdc.gov/ncidod/dvrd/vcjd/
National Center for Infectious Diseases, Centers for Disease Control and Prevention
Since 1996, evidence has been increasing for a causal relationship between ongoing outbreaks in Europe of a disease in cattle, called bovine spongiform encephalopathy (BSE, or 'mad cow' disease), and a disease in humans, called variant Creutzfeldt-Jakob disease (vCJD). Both disorders are invariably fatal brain diseases with unusually long incubation periods measured in years, and are caused by an unconventional transmissible agent.