Professor Stanley Prusiner's team made the discovery

This starts to create a mess of badly-folded prions littering nerve cells, which will eventually start to interfere with its function, and then kill it.

It is thought that the body might normally have the ability to clear these mis-shaped proteins - but if, for some reason, it lacks this ability, whole swathes of cells are destroyed by the rogue prions.

This creates the tell-tale "spongy" appearance of the brain which characterises both BSE and vCJD.

Many scientists believe that the prions which eventually produced vCJD entered the human body when they were exposed to BSE-infected meat, and the rogue prions it contained.

Twin problems

The researchers need to do two things to make an anti-CJD drug work.

From week to week, patients can experience ups and downs. But the fact that there has been an improvement does encourage hope

Professor Alan Colchester, neurologist

They need to find a way of clearing out the rogue prions from cells - and a way of doing this with a drug whose molecules are small enough to squeeze through the filter which protects the brain from poisons carried around in the blood.

This is called the blood-brain barrier.

The disease can only be diagnosed after death

In the test tube, they looked at mouse cells deliberately infected with prions , and found that two drugs, quinacrine, an anti-malarial, and chlorpromazine, an anti-psychotic, were promising.

They seemed to be able to stop the prions being converted from the right shape to the wrong shape.

In effect, the drug was able to clear out infected cells - and keep them cleared out.

Chlorpromazine makes it across the blood brain barrier easily, quinacrine less so, although quinacrine was more effective at prion "clearance".

Big questions

To demonstrate this in a few cells in laboratory conditions is one thing - to do it in complex human patients is something completely different.

And a big question-mark remains - will the brain, even if successfully cleared of rogue prions, be able to stage a repair-job on the nerve cells already left damaged by the assault?

If it can't, then the most patients can hope for is a slowing or halting of the progress of the disease.

Even if it can, it is far from certain whether a complete recovery could ever be possible.

Does it work?

The evidence for this is extremely patchy.

The apparent success enjoyed by Rachel Forber, while encouraging, stops a long way short of proving the drugs are effective.

Indeed, a second patient given the drugs showed no sign of improvement whatsoever.

And it has yet to be discovered whether the improvement seen in Rachel will continue over time.

Professor Alan Colchester, an experienced UK neurologist, told the BBC: "From week to week, patients can experience ups and downs. But the fact that there has been an improvement does encourage hope."

One of the central problems faced by scientists is that there is currently no reliable way of proving that a particular patient has vCJD.

While the symptoms are devastating, and strongly suggest vCJD, it is still an extremely rare disease, and a number of little-understood neurodegenerative disorders share some of its features.

So doctors do not know if they are producing an improvement in a vCJD patient, or in a patient with some other disorder.

What is needed now is wider-scale testing on larger numbers of suspected vCJD patients.

Despite the high profile given to the disease by the media, there are still relatively few of these worldwide. Recruiting sufficient numbers into a clinical trial will be difficult.

Professor Colchester said: "We have got to brace ourselves for a period of uncertainty - at least for months, or, I think honestly, for years."

"It's too early to be optimistic."

However, one advantage of these treatments is that they are established drugs.

Their side-effects and safety are well known, meaning that at least one lengthy section of the clinical trials can be bypassed.

The future

Whether or not these drugs, or drugs like them, can tackle vCJD in humans, scientists will still learn much from the Californian work.

They believe they have identified which part of the drug molecules could be responsible for the effect on prions.

And they believe that refining this could even help patients with other diseases which may have their roots in the "clogging" of brain cells, such as Alzheimer's Disease.