Creutzfeldt-Jakob disease (CJD) is an illness of the nervous system that causes damage to the brain. (The disease is named after 2 German scientists). CJD is fatal, and there is no known cure. CJD is caused by an abnormal protein - a prion - that then contaminates the nervous system.

In some respects a prion is similar to a virus as it can replicate and cause disease. Unlike a virus, it is made entirely from protein and has no genetic material. This makes prions much 'tougher' than viruses or bacteria. They can survive extremes of heat and radiation. Antibiotic or anti-viral medicines have no effect on prions.

There are four different types of CJD.

Sporadic CJD

Sporadic CJD accounts for 85% of CJD cases, yet remains very rare. It is estimated that one death in every million is caused by sporadic CJD. What triggers sporadic CJD is unknown, but it normally affects people over 40 years of age.

Iatrogenic CJD

This type of CJD is the result of the infection being spread from someone with CJD through medical or surgical treatment. The majority of iatrogenic CJD cases happened through the use of human growth hormone, which is used to treat children of restricted growth.

Between 1958 and 1985, thousands of children were treated with the hormone, which at the time, was extracted from the pituitary glands (a gland that sits at the base of the skull) of human corpses. A tiny minority of those children developed CJD, as the hormones they received were taken from glands infected with CJD. Since 1985, all human growth hormone is artificially manufactured, so there is now no risk.

A few other cases occurred through receiving transplants of infected tissue, or coming into contact with surgical instruments that were contaminated with CJD. This happened because prions are 'tougher' than germs or bacteria, so the normal process of sterilizing surgical instruments had no effect. Once the risk was recognized, the guidelines on organ donation and the re-use of surgical equipment were tightened.

As a result, cases of iatrogenic CJD are now extremely rare. There were 6 deaths from iatrogenic CJD in 2006, though it is thought that the original infection could have happened up to 20 years ago.

Familial or genetic CJD

This is a very rare form a CJD caused by an inherited mutation of the gene that produces normal proteins. The altered gene seems to produce prions that then cause CJD. 

Variant CJD (vCJD)

A new type of CJD, new variant CJD, now referred to simply as variant CJD (vCJD) was identified in 1996. Unlike sporadic CJD it affects mainly people in their mid twenties.

There is clear evidence that variant CJD is caused by the same strain of infection that causes what is known as mad cow disease (bovine spongiform encephalopathy or BSE) - a prion.

A government inquiry in 2000 concluded that the prion was spread through cattle feed, and had entered the human food chain. The evidence suggests that the most likely explanation for variant CJD cases is exposure to that prion (but there is no known link to sporadic CJD).

Strict controls have been in place since 1996 to prevent BSE from entering the human food chain

Cases of variant CJD seemed to have peaked during the years from 1996 to 2003. In 2000 there were 28 deaths from variant CJD, while there were only 5 in 2006. Some experts believe that the food controls have worked, and that further cases of variant CJD will continue to decline.

Other experts have warned that the people who died could have had a genetic trait that meant that the variant CJD affected them more quickly than normal. Other similar infections caused by prions normally take between 15 and 20 years before they become active.

They argue that many people could also have variant CJD, but the symptoms might not begin to show for many years to come.

All cases of CJD are carefully monitored by the National CJD Surveillance Unit.