Mad cow disease is a fatal disease that slowly destroys the brain and spinal cord in cattle, also known as bovine spongiform encephalopathy (BSE) . People cannot get mad cow disease, but in rare cases, they may get a human form of mad cow disease called variant Creutzfeldt-Jakob disease (vCJD), which is fatal. vCJD destroys the brain and spinal cord over time, and it is believed to be caused by eating beef products contaminated with central nervous system tissue, such as brain and spinal cord, from cattle infected with mad cow disease. The infectious agent that causes mad cow disease is an abnormal version of a protein normally found on cell surfaces, called a prion. Prions are found in the brain, spinal cord, and small intestine of affected cows, but there is no proof that prions are found in muscle meat or in milk. If a person eats nerve tissue from an infected cow, they may not feel sick right away, and the time it takes for symptoms to occur after exposure to the disease is not known for sure, but experts think it is years. There is no cure for vCJD, and treatment includes managing the symptoms that occur as the disease gets worse.

It is important to note that classic CJD, a rapidly progressive, invariably fatal neurodegenerative disorder, is not related to mad cow disease. Classic CJD has no known cause and occurs each year at a rate of one to two cases per 1 million people throughout the world, including in the U.S. and countries where mad cow disease has never occurred. It is not linked to eating nerve tissue from mad cow disease-affected cattle, and both vegetarians and meat eaters have died from classic CJD.