Using harmless pieces of DNA they received through the post, researchers at the State University of New York built a synthetic version of the polio virus so like the real thing that it infected mice and made them ill.

The World Health Organisation is mounting a campaign to eradicate the wild polio virus from the world by 2005. Last month Europe was officially declared free of the disease.

The New York team's work, published yesterday in the online version of the journal Science, alarmed polio specialists, but also prompted concerns that other viruses more suitable for biowarfare, such as smallpox, could be synthesised.

Up until now, fears over the use of smallpox in bioterrorism in the wake of September 11 have been tempered by the hope that the virus only exists in two labs in the world, one in Russia and one in the US.

"It is a little sobering to see that folks in the chemistry lab can basically create a virus from scratch," James LeDuc, a virus specialist at the Centres for Disease Control and Prevention in Atlanta, told Science.

Viruses are on the borderline between living and non-living things. Like living things, including us, they encode their physical characteristics in DNA (in the case of polio, a related substance called RNA) and produce copies of themselves.

Unlike living things, they consist of practically nothing else except that chemical code. They have to hijack the cells of living organisms in order to reproduce.

For decades, it has been theoretically understood that all that was required to synthesise a virus was to turn the written version of its DNA or RNA code into real, chemical DNA or RNA. Now, someone has done it.

Written down, the genetic code for polio is only 7,741 letters long.

Even so, it took Jeronimo Cello, Aniko Paul and Echkard Wimmer two years to put it together chemically using short stretches of DNA ordered from private companies that make them.

Smallpox is much longer and more complicated. Written down, it would run to 185,000 letters, about as many as a small book.

But Vadim Agol, a virologist at the Russian Academy of Medical Sciences in Moscow, told Science: "In principle, yes, it's possible to synthesise smallpox."