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. One theory is that the disease is caused by a change in the shape of some of the proteins found in animal cells. This change may be caused by other abnormal proteins called prions. In affected cows, the abnormal proteins (prions) are found in the brain, spinal cord, and small intestine. Another theory is that mad cow disease is caused by a virus that causes the proteins to change and become abnormal (prions).
WebMD Medical Reference from Healthwise