Each year around the world there are still up to 2,000 cases of the disease, which is spread by fleas on rats.

If the plague vaccine reportedly developed at Britain's military research centre Porton Down is a success, there could be the obvious benefit for those thousands.

Bubonic plague spread by fleas on rat

But it is fear of the plague being used as a biological weapon which has led to the research taking place.

International conventions should prevent countries from developing chemical and biological weapons.

But there have been well publicised cases of breaches of the convention, including this week reports that the Yugoslav army allegedly used nerve agents such as sarin on Kosovo Albanians.

Iraq is known to have produced mustard gases and a deadly nerve gas called VX. In August 1988, its forces used both chemical and gas munitions against Kurdish civilians in Iraqi Kurdistan.

United Nations weapons inspectors have discovered and destroyed large quantities of both chemical and biological weapons in Iraq.

Biological materials included:

• botulinum,which would cause botulism
• anthrax, which attacks the skin and lungs
• aflatoxin, which causes liver damage and cancer, and
• clostridium, which can cause tetanus and botulism

The mastermind behind Iraq's biological weapons programme was thought to be Dr Rihab Taha, dubbed "Dr Germ" by the media in the West. It was of some embarrassment to the UK government that she took her doctorate in plant toxins at the University of East Anglia between 1981-84.

Main worries

Dr Phillip Towle of the Centre for International Studies at Cambridge University said it was the plague and anthrax about which there had been most talk.

Pneumonic plague sufferers in India

But as weapons neither would be particularly useful because they would be difficult to control, he said.

"They are spread around by being inhaled, so anyone using them would have to try to create a plume of gas going through the air," he said.

This would be of limited use because of its indiscriminate nature.

"The real future danger is the development of genetically specific weapons," he said. For instance a germ which would attack particular racial groups but leave others unharmed. There have been reports of developments of such germs.

But he added: "I suspect that Iraq is not quite there. It's been talked about, but my guess is that it might be possible to develop them in hi-tech organisations like Porton Down itself, if they really worked on it."

122mm Iraqi rockets containing sarin, destroyed after the Gulf War

The attraction for various countries in developing chemical and biological weapons would be to counter nuclear arms. Indeed biological weapons have been referred to as "the poor man's nuke".

So some of Israel's neighbours, knowing that it had nuclear weapons, might want to develop biological weapons. This could include Iraq, and conceivably Libya and Iran as well, Dr Towle said.

One reason countries might choose to develop them is that they would not be as visible to outsiders as nuclear arms. "Especially from the air, a laboratory is much easier to hide," he said.