Creutzfeldt-Jakob disease (CJD) is a brain wasting disease. The best evidence is that it is caused by a malformed protein that interferes with the repair and operation of existing brain tissue. It appears to be related to Mad Cow disease and scrapie, both of which cause similar symptoms in cows and sheep respectively. Brains of Creutzfeldt patients on autopsy are riddled with holes.
The mode of transmission of the disease is believed to be consumption of meat contaminated by brain tissue. Prions can apparently survive heating, so cooking does not appear to affect transmission of the disease. However, vegetarians have also been known to have developed the disease. The disease is very rare, even in countries where Mad Cow disease is known.