Mad cow disease is the common name of "Bovine spongiform encephalopathy". It's a disease that affects a cow's nervous system and their physical state. It comes from the chemistry structure of prion protein. When cattle get this disease, they have holes forming in their brains. The brain becomes spongy and begins to waste away. The cattle become very violent and show signs of mental disorder and have trouble when they move. It has a long incubation period, about 4~5 years. Cattle that are 4~5 years old can get the disease very easily.