British scientists say they have made a breakthrough in their development of a bird flu drug that could treat humans in the event of an outbreak.
However they warn that it could be another five years before a final version of the drug is ready to be used to treat the H5N1 virus.
At present the H5N1 strain of bird flu remains a disease predominantly of birds and all of the 239 cases of human infection to date, have been from direct contact with infected birds.
Though scientists insist there is as yet no evidence the virus is mutating, which would enable it to transfer between humans, there is concern this could happen.
As many as 60 percent of those infected have died, and the best known drugs to tackle H5N1 infection in humans are oseltamivir known as Tamiflu and zanamivir known as Relenza, both of which were originally developed to fight other forms of human flu.
A team of scientists led by John Skehel of London's National Institute of Medical Research say they have found a chink in the armour of the H5N1 virus that could lead to more potent drugs against the deadly virus.
The H5N1 virus spreading around the globe has a surface protein which helps the virus penetrate and infect cells.
This protein is called neuraminidase, the N1 in H5N1 and is the enzyme targeted by the drugs Tamiflu and Relenza, which are being stockpiled by some countries in case the H5N1 bird flu becomes a human pandemic.