PRIONS, the biological villains involved in BSE, vCJD and other similar diseases, are rogue proteins that are found in the brains of people with vCJD. Prions encourage nearby proteins to misfold; these turncoat proteins then do the same. The result is a domino chain of damage which wrecks brain tissue and turns it into a sponge.

Prions are thought to be the culprit in transmissible spongiform encephalopathies (TSEs), a class of neurological diseases that affects human beings and several species of animal. The oldest is scrapie, first noted in sheep in the 18th century. BSE (bovine spongiform encephalopathy) was first described in 1986; it is likely to have originated in cattle via feed infected with scrapie. This showed that TSEs could jump the species barrier.

Before 1995, there were three types of Creutzfeldt-Jakob disease. One, genetic or familial CJD, is a rare inherited condition that kills a handful of people each year. An equally rare type is iatrogenic CJD, which arises as a result of medical or surgical procedures. Some children who had treatment with human growth hormone in the 1980s developed it. Sporadic CJD is the most common type — it strikes one person in a million, has no obvious cause, is seen worldwide and generally affects middle-aged or elderly people.

In 1995, scientists noticed a fourth type of CJD with its own neurological signature. Eventually, they concluded that this new form of CJD, seen only in Britain, had appeared in people who had eaten BSE-infected meat. Once again, a TSE had jumped the species barrier, this time from cattle to human beings. This new form of CJD was named variant CJD, or vCJD, to distinguish it from the others. Its progression is generally slower than that of sporadic CJD, although the symptoms are as distressing: loss of co-ordination and speech, gradual muscle failure and eventual death.

The vast majority of vCJD sufferers have been in their teens or twenties (the average age is 27). The condition is twice as prevalent in the North as in the South (an effect attributed to diet), but affects men and women equally. A definitive diagnosis can be made using brain tissue, or tonsil tissue. Scientists are working on a blood test. There is no cure for any of the CJDs.