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).