By Richard Black
BBC World Service science correspondent
A new study suggests that bird flu is becoming more dangerous each year.
The avian flu virus continues to evolve
Scientists in China and the US injected mice with samples of avian flu virus which emerged in different years.
They found that the newer forms of the virus kill more rapidly than their predecessors. The fear is that this will increase the risks to humans too.
The researchers, whose work is reported in Proceedings of the National Academy of Sciences, say more action is needed to try to curb the spread of the virus.
Earlier this year a strain of bird flu called H5N1 killed 23 people in east Asia - the most in any year since the strain first emerged seven years ago.
Now scientists have found evidence that it is mutating in ways which make it more lethal.
They took 21 different samples of H5N1 isolated over the years since 1997, and injected them into mice.
A strong pattern emerged - the most recent viruses kill the mice much faster - which would suggest bird flu is also becoming more lethal to humans.
However, as Professor Robert Webster, from St Jude Children's Research Hospital in Memphis, US, points out, it has not yet acquired the most dangerous mutation.
He said: "The thing that's lacking so far in these viruses is the ability to transmit human to human.
"And that's the trick that we hope they don't achieve; but if they do, then we have very big problems."
Professor Webster's team has been able to document some of the genetic changes in the virus and understand what they do - for example enabling it to evade bits of the human immune system.
However, other mutations remain a mystery. Professor Webster believes the mutations may be caused by the virus jumping from species to species.
As well as becoming more lethal, H5N1 is also now able to reproduce in more parts of the body than before.
The trend towards more dangerous forms of the virus is one which the researchers believe will continue.
The concern is that the virus will eventually accumulate enough genetic changes to become good at passing between humans.
Even more of a concern would be the sudden change that could be caused should the flu combine with a human flu in someone's body.
The two viruses could swap genes and create a potent hybrid as deadly as the bird
strain and as contagious as a regular human strain.
Professor Webster, who runs a World Health Organization-affiliated laboratory studying animal diseases, believes the international community needs to put more resources into tracking bird flu if a major epidemic is to be avoided.
"WHO have not adequately addressed the human/animal interface. It's difficult to do; I think it's on their radar screen now," he said.
The avian flu has forced authorities to slaughter millions of chickens and other fowl in Asia to stem outbreaks in recent years.