CJD (Creutzfeldt-Jakob Disease) is a rare illness and is one of a group of diseases called prion diseases, which affect humans and animals. Prion diseases exist in different forms, all of which are progressive, currently untreatable and ultimately fatal. Their name arises because they are associated with an alteration in a naturally occurring protein: the prion protein. CJD was first described in 1920. The commonest form is called sporadic CJD and occurs worldwide causing around 1-2 deaths per million population per year. A new form of CJD (variant CJD) linked to bovine spongiform encephalopathy (BSE) in cattle was identified in 1996. There are also genetic forms of human prion disease linked to mutations of the prion protein gene and cases caused by infection via medical or surgical treatments (iatrogenic CJD).