Mad cow disease is the common term for Bovine spongiform encepholopathy (BSE), a progressive neurological disorder of cattle. It is caused by prions. Symptoms include an excitable or nervous temperament to external stimuli such as touch to the skin. A prion (short for proteinaceous infectious particle) is a unique type of infectious agent, as it is made only of protein. Prions are abnormally-structured forms of a host protein, which are able to convert normal molecules of the protein into the abnormal structure.