Deadly bird flu is mutating to spread from person to person, bringing a disastrous global pandemic closer, experts fear.
Evidence from Southeast Asia suggests that the virus, which could kill tens of millions of people worldwide, is becoming less virulent, but at the same time more infectious to people.
Death rates from the virus have plunged in northern Vietnam, says the World Health Organisation (WHO), though it is still killing more of its victims than any previous outbreak.
The instances where it appears to have spread from person to person are rising.
Six weeks ago, the Independent on Sunday revealed that the Government had told mortuaries and emergency services to prepare for up to 750,000 deaths from the disease in Britain. Flu pandemics occur when three developments take place: a virus emerges to which humans have little or no immunity; it is able to infect people, and it mutates to spread efficiently among them.
The bird flu virus - codenamed H5NI - has crossed the first two barriers, and experts fear it is now about to breach the third.
"Suddenly it’s a very different virus that might suddenly become extremely transmissible," said Peter Horby, of the WHO office in Hanoi.
He said that it was impossible to predict when that might happen, but there were "a number of indications" that the virus was already becoming more dangerous.
Ironically, one of the main ones is that the virus is becoming a less ruthless killer. By allowing more of its victims to survive, it enables them to live to infect other people.
Up to now about 70 per cent of those infected have died. But the WHO reports that the death rate in northern Vietnam has now fallen to 20 per cent, though it has remained the same in the south.
Even the reduced level would make it much the worst flu pandemic to hit the world. "Spanish flu", which killed 40 million people in 1918, had a mortality rate of just 5 per cent.
A second ominous indication is that the flu has been widening its targets in northern Vietnam. Previously it mainly attacked children and young adults, but now it is affecting all age groups.
Then, while almost all previous victims have been infected directly by chickens or ducks, there is a steadily increasing number of clusters of disease, where it appears to have spread between people.
There are now at least seven of these, almost all in the northern province of Haiphong. WHO officials say this is unprecedented.
Finally, the virus itself seems to have changed physically. Vietnamese health experts say it has evolved in the north of the country by dropping an amino acid.
The US Government’s Centers for Disease Control and Prevention, which has analysed many specimens of the virus from Vietnam, adds that new strains of it "are becoming more capable of causing disease for mammals".
It and the WHO stress that there is no conclusive proof yet that the virus is spreading efficiently between people.
But scientists are agreed that it is only a matter of time before it does and that, then, jet travel will spread it rapidly around the globe.
Bird flu and NZ
* An outbreak of one of the new flu viruses here could cost 3700 lives and put 20,000 more people in hospital.
* More than a million New Zealanders would need to visit a doctor
Source: Ministry of Health, March 2005