The following correction was printed in the Guardian's Corrections and clarifications column, Saturday March 24 2007

As well as the two forms of plague, bubonic and pneumonic, described in this report, there is a third. The septicaemic form occurs when infection spreads directly through the bloodstream.

The following correction was printed in the Guardian's Corrections and clarifications column, Thursday March 22 2007

In the report below, we said that in recent decades the World Health Organisation had recorded outbreaks of the plague in 125 countries. We meant to say "in 25 countries". This has been amended.

A multiple drug-resistant form of the plague, one of the oldest and most lethal diseases in human history, has been identified by scientists, prompting fears of devastating future outbreaks that cannot be contained by antibiotics.

Tests on a strain of the disease-causing bacterium, Yersinia pestis, taken from a 16-year-old boy in Madagascar revealed the organism has developed resistance to eight antibiotics used to treat the infection, including streptomycin and tetracyclin.

The bacterium is believed to have become resistant to drugs after swapping genes with common food bacteria such as salmonella, E coli and klebsiella, probably while being carried in the guts of fleas, which spread the disease by biting infected rodents.

The discovery has alarmed scientists who fear multiple drug-resistant strains of the plague may emerge in other countries, leading to highly dangerous pandemics which spread rapidly. Another serious concern is that drug-resistant strains of the organism may be collected by terrorist organisations and released into the air, causing widespread infection.

The plague first emerged several thousand years ago and swept across Asia and Europe during the Black Death pandemic between the 14th and 17th centuries. Successive pandemics are estimated to have claimed some 200m lives. Antibiotics brought the disease under control, but in recent decades the World Health Organisation has recorded outbreaks in 25 countries, most recently in the Democratic Republic of Congo, which last year reported 1,174 suspected cases and 50 deaths.

There are two forms of plague. Infections caused by bites lead to bubonic plague, named after large swellings or buboes on the skin. Untreated, it kills 40% to 70% of people, usually in less than a week. The second form, pneumonic plague, is almost 100% fatal within days if not treated.

The last fatal case of the plague in England was at the end of the first world war, when a Mrs Garrod died on June 19 1918 in Suffolk, a week after a neighbour, Mrs Bugg, succumbed to the infection.

A team lead by Timothy Welch at the US department of agriculture analysed genetic sequences from the drug-resistant plague microbe and compared them with similar sequences from food bacteria. They found a nearly identical 180-gene segment responsible for drug resistance in all of the organisms.

"The resistance is carried by these genes and they can transfer easily from one strain to another with very high efficiency," said Elisabeth Carniel, who co-authored the paper in the journal Public Library of Science One. Although the bacteria used in the study were typically found in meat and poultry, they also thrive in soils.

"These kinds of multiple drug-resistant bacteria are spreading in the environment, so the chances are that contact between them and the plague bacterium will be higher than we thought. Plague is not the disease it was because of the existence of antibiotics, so if we can't use antibiotics any more, or if we have only a very limited range of drugs that work, that will make the disease much more dangerous."