Mad cow disease (bovine spongiform encephalopathy, or BSE) is a degenerative, usually fatal disease that affects the central nervous system of cattle, sheep, and goats. While humans cannot get mad cow disease, in rare cases they may get a human form called variant Creutzfeldt-Jakob disease (vCJD) if they eat nerve tissue (the brain and spinal cord) of cattle infected with mad cow disease.
No one is sure what causes mad cow disease. Experts believe that the disease may be caused by infectious proteins called prions. In affected cows, these abnormal proteins are found in the brain, spinal cord, and small intestine.