Creutzfeldt-Jakob disease is a rare, incurable disease of the brain that causes physical and mental deterioration, progressing to death usually within a year of onset. Now known to be caused by an abnormal protein called a prion, it is one of a group of diseases which also includes scrapie in sheep and bovine spongiform encephalopathy (BSE) – "mad cow disease".

Normally CJD attacks older people, but in Mar 1996 came the first reports of young people in the UK dying of a variant of the disease. Eighteen months later scientists confirmed that the agent giving rise to the new variant (nvCJD) is identical to that of BSE, confirming the link between the human disease and the consumption of infected beef or beef products.