The breakthrough will help future work on treatments for a disease that has killed millions of people throughout history, and which is viewed as a possible weapon of bio-terrorism.

The plague bacterium, Yersinia pestis, is a dynamic and rapidly changing organism.

In a very short space of time in evolutionary terms, Y. pestis acquired the ability to leap between fleas and mammals, and to live in the blood, instead of the intestine.

It also acquired the ability to cause potentially fatal swelling, coughing and haemorrhaging.

Genetic structure

The genetic structure of the bug has been worked out by scientists at the Sanger Centre in Cambridge, UK, led by Dr Julian Parkhill.

They have pinpointed some the genes that Y. pseudotuberculosis gained that enabled the bug to turn into a killer. These include genes that manufacture toxins that enabled the bug to infect fleas.

The scientists also discovered a raft of genes no longer needed in the new environment that are deactivated in Y. pestis.

Slow process

They also found that the plague bacterium had a similar genetic structure to the bacterium that causes leprosy. They also uncovered evidence that Y. pestis is still evolving, and may one day develop into an organism that poses no threat to the cells of its host.

However, scientists Dr Stewart Cole and Dr Carmen Buchrieser, from the Pasteur Institute in Paris, France, warn that even if the plague is slowly dying out, the process will take a very long time.

They say: "Meanwhile, the information provided by the Y. pestis genome sequence should be applied to ensure that plague does not re-emerge, and that one of the potential weapons of bio-terrorism can be neutralised."

Details of the gene map are being posted on the internet so the information is freely available to researchers around the world.

Rick Titball, technical manager of microbiology at Porton Down, said: "The benefits of making this kind of information publicly available greatly outweigh the risk of someone getting it and using it for nefarious purposes.

"It is much more valuable to someone who wants to develop vaccines or antibiotics By putting this information out we can help accelerate research, especially in developing countries where they might not have sophisticated equipment.

"Indeed, sequencing Y. pestis has helped development of a vaccine which is currently undergoing clinical trials."

Europe ravaged

The plague, otherwise known as the Black Death, ravaged Europe and Asia between the 14th and 17th Centuries. In the 14th Century alone it is estimated to have killed 200 million people.

Although the disease is no longer a major health problem in Europe, it is still prevalent in some parts of the world. About 3,000 cases are reported annually to the World Health Organisation.

There are fears that climate change and increasing globalisation could see a re-emergence of the disease in the developed world.

Black rats, the bacterium's hosts, have recently reappeared in some parts of the UK.

The research is published in the journal Nature.