Flu could be a far more dangerous bioterror weapon than smallpox or anthrax, scientists have warned.
Different flu strains emerge each year
Writing in the Journal of the Royal Society of Medicine, scientists said information from the sequencing the genome of the 1918 flu epidemic, which killed between 20 and 40 million people, could be misused by terrorists.
They warn that once the sequencing is complete, unscrupulous scientists could use the information to create more virulent strains.
The fact that it should be possible to transmit flu in aerosol form also increases its attractiveness as a biological weapon, they warn.
The US team warn that flu could be even more deadly than previously thought.
I don't think it could be used in that way now
Professor John Oxford, Queen Mary's School of Medicine, London
It has been estimated that flu kills 20,000 people a year in America.
But the researchers say the number could be over four times that figure because of a link between flu and fatal heart attacks.
The team, from the University of Texas Health Science Center, were looking at the links between flu and cardiovascular disease when they became concerned about how genetic information about the flu virus could be misused.
Dr Mohammed Madjid, who led the research, warned there were particular concerns about how influenza could be used as a bioweapon.
He said the fact it was so common would make it easier for terrorists to obtain the virus.
But the ordinariness of flu would also make it more difficult for experts to identify clusters of cases, so an epidemic could be well established before it was detected, they said.
If an epidemic should take hold, the researchers say it would be difficult to immunise against because the incubation period is short.
They also warn flu is very difficult to eradicate since birds, rats and pigs all carry the virus.
Dr Madjid and his colleagues say bodies such as the World Health Organization and the US Centers for Disease Control should co-ordinate experts in flu, bioterrorism, health policy, international law and ethics to look at the issue.
They also call for better security in laboratories, stockpiling of antiviral drugs and more work on developing vaccines.
The researchers also call immunisation programmes to be improved and better disease surveillance.
Dr Madjid told BBC News Online: "Using influenza as a bioweapon is a probability.
"It's just a matter of technology. If it's difficult now, it will be easier in six months and much easier in a year's time."
"The question is about preparation. We can't wait until something happens and then say 'what do we do'.
Professor John Oxford, a virologist at Queen Mary's School of Medicine, London, leads one of two teams sequencing the 1918 influenza virus.
He told BBC News Online even if they were able to pinpoint why the virus killed so many, it would be difficult to use the information to create a bioweapon.
"I don't think it could be used in that way now.
"It would need a lot of expertise and a huge teams of people to enhance its virulence."
Professor Oxford said his team, and another working in America, were about two years away from completing the genome's sequence.