Answer:
Bovine spongiform encephalopathy (BSE, commonly referred to as mad cow disease) is caused by a prion. This is a normal protein found primarily in the brain and spinal cord of cattle that gets folded the wrong way. Once one protein is mis-folded, it starts to cause other copies of the same protein to also misfold - as these mis-folded proteins accumulate, they eventually choke the nerve cell until it dies. The clinical signs of BSE - sudden aggression, change in gait, etc. - are caused by the various nerve cells dying.