'Mad cow disease' is properly known as BSE (Bovine Spongiform Encephalopathy). It destroys the grey matter of the brain and spinal cord of cattle, making the brains look like a sponge. It was first seen in the United Kingdom in 1986. The cause is not an infection like viruses/bacteria, but the development of a self-replicating protein, known as a 'prion'. These prions get in the way of normal functions. Prions are highly stable and capable of surviving freezing, drying and heating at temperatures used for pasteurising and sterilizing.

BSE is similar to 'scrapie' in sheep and Creutzfeldt-Jakob Disease (CJD) in people. While these diseases look similar, they may or may not be related. There is still no proof that any of these diseases can jump species creating similar effects in other animals/humans.

BSE has a long incubation of four to five years and usually proves fatal within weeks of the symptoms emerging. The symptoms are hypersensitivity to external stimulus, lack of coordination, wasting and often death. The disease has a long incubation period, from 2 to 8 years, so is mostly seen in adult cattle - over 30 months old.

The UK government and popular opinion considers that BSE was caused by the nasty habit of cattle eating rendered remains (called Meat and Bone meal or MBM) of sheep and other cattle. However, some people believe BSE prions may be caused by other factors inherent in intensive farm systems, such as pesticide or other chemical applications. They point to the fact that many calves have been born with BSE in the UK despite the ban (1996) on feeding the animal parts. These are called BARBs - Born after the Re-inforced Ban - calves.