vCJD produces gaps inside the brain

It seems to be capable of developing in much younger people than standard CJD.

While death from standard CJD happens approximately six months after diagnosis, it generally takes much longer - up to 18 months - for a patient with vCJD to die.

Diseases like vCJD and BSE are believed to be caused by an infectious agent called a prion, which can be carried in blood and tissue.

This causes the death of brain cells, in humans causing symptoms such as unsteadiness, insomnia, memory loss and dementia.

Post mortem

Eventually, enough brain cells die for the tell-tale gaps to appear in the tissues, although the only way of definitively confirming a CJD case in humans is following death during post mortem.

The prion is a protein, thought to linger in human tissue, potentially for many years, before any symptoms begin to emerge.

Unlike a virus or bacteria, it contains no genetic information of its own, and does not spread by reproducing or making copies of itself.

Instead, scientists believe that it "recruits" another natural protein, PrP, which sits on the surface of brain cells, to become like it.

Eventually, huge numbers of these mutated proteins build up, form insoluble masses within cells, which kill them.

While no definitive link between prion diseases in animals and CJD in humans has been established, the mechanism and progression of the diseases are so similar that most scientists are convinced that infection with BSE prion may under certain circumstances lead to the development of vCJD in humans.

In addition, scientists have been able to produce spongiform symptoms in mice injected with diseased brain matter from cows.

Recent studies also showed that certain cells contained in blood were capable of harbouring the infectious agent and causing spongiform disease when transfused into another animal.

When a cow has BSE, certain parts of its carcass harbour far larger concentrations of prions than others.

Cuts of meat

The brain and spinal cord are thought to be the main reservoirs for prions.

It is the belief of scientists that, just as BSE is likely to have been passed from infected cow through feed contaminated with brains and spinal cord, humans can acquire the BSE prion through cuts of meat which contain this material.

Again, this has yet to be satisfactorily proven by experts.

Certain cuts of beef are less likely to harbour the prions

And the obvious question is that while millions of UK children and adults have potentially consumed infected flesh from cattle, only a handful so far have succumbed to the disease.

Doctors do not yet know the likely incubation period for CJD.

There have been recent strides in diagnosing the condition, through better imaging with brain scans, and through tests on tonsils, which may also harbour the prions.