A disease of humans characterized by dementia and destruction of brain tissue. It is now known to be caused by an abnormal prion protein and is transmissible, although there is also an inherited familial form. This rare disease typically affects middle-aged and elderly people and leads to rapid mental deterioration and death. The abnormal prion interferes with the structure of normal prion protein in brain tissue, resulting in accumulations of the protein and consequent tissue damage. In most cases the source of infection is unknown. However, it is well established that infection can result, for example, via injections of growth hormone derived from infected human cadavers. During the 1990s a novel form of the disease emerged, called ‘variant CJD’, which typically affects young healthy individuals. This is thought to be caused by consumption of beef products derived from cattle infected with bovine spongiform encephalopathy or transfusion of blood from infected donors.