Rare fatal disease of the central nervous system. It destroys brain tissue, making it spongy and causing progressive loss of mental functioning and motor control. The disease commonly arises in adults between the ages of 40 and 70. Patients usually die within a year. There is no known cure. The disease is caused by a prion that builds up in neurons. Inherited or random mutation accounts for 99percnt of cases; the rest come from prion exposure during medical procedures and possibly from eating the meat of cattle with mad cow disease.

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