By Dr David Whitehouse BBC News Online science editor

Scientists have assembled the first synthetic virus.

The US researchers built the infectious agent from scratch using the genome sequence for polio.

Scientists are divided about whether a virus is alive. For those that think it is, then this synthetic artefact would constitute a simple form of life.

Responding to criticisms that such research could lead to bioterrorists engineering new lethal viruses, the scientists behind the experiment said that only a few people had the knowledge to make it happen.

'New reality'

To construct the virus, the researchers say they followed a recipe they downloaded from the internet and used gene sequences from a mail-order supplier.

The animals were paralysed and then died.

"The reason we did it was to prove that it can be done and it now is a reality," said Dr Eckard Wimmer, leader of the biomedical research team and co-author of the study published in the journal Science.

"This approach has been talked about, but people didn't take it seriously," said Dr Wimmer.

"Now people have to take it seriously. Progress in biomedical research has its benefits and it has its down side. There is a danger inherent to progress in sciences. This is a new reality, a new consideration."

'Very easy to do'

According to researcher Jeronimo Cello, the polio virus assembled in the laboratory is one of the simplest known viruses. "It was very easy to do," he said.

Dr Wimmer added: "The world had better be prepared."

Smallpox has been eradicated in the wild, but specimens are stored in the United States and in Russia.

Dr Wimmer said assembling the polio virus showed that eradicating a virus in the wild might not mean it was gone forever because biochemists could now reconstruct those viruses from blueprints.

Following last year's terrorist and anthrax-by-mail attacks, US officials became concerned about the threat of smallpox and arranged for the manufacture of enough vaccine to protect the US population.

Matter of time

Dr CJ Peters, director for the Center for Biodefense at the University of Texas Medical Center at Galveston, said experts had known for years that it was theoretically possible to assemble a virus in the lab.

Dr Peters said he was concerned that publicity about a synthesized virus might lead some people to believe "that there is nothing that can be done about bioterrorism - which is not the case".

He added that it was possible that viruses like Ebola could be assembled in laboratories, but there were only a few people in the world with that skill.

Polio is on the brink of being eradicated worldwide and there are plans to stop inoculations against the disease after it disappears from nature.

Dr Wimmer said that this policy should be reconsidered. Stopping vaccination could lead to a generation of people highly susceptible to polio, enhancing its appeal as a weapon.

The World Health Organization is planning to stockpile vaccines against a return of polio and Dr Wimmer said that policy should be followed everywhere.