World Health Organisation scientist Hitoshi Oshitani spends his days planning for a nightmare scenario - a bird flu pandemic among humans that would kill millions and bring nations to their knees.

There is much that experts still do not know about the deadly H5N1 strain of avian influenza - exactly how humans contract it from infected poultry, and why so many of its victims are healthy youngsters.

But Oshitani says they do know vulnerable countries are ill-prepared and that if the virus mutates and erupts among humans in one of Asia's crowded mega-cities, it will be impossible to prevent it from becoming a pandemic.

"If a pandemic starts we cannot do anything to stop it. What we can do, once a pandemic starts, is just to reduce the negative impact by being better prepared," said the Manila-based WHO policymaker.

"It's probably just a matter of time. Every 30 to 40 years we have had a pandemic," he told AFP on the sidelines of an international conference on bird flu held this week in Kuala Lumpur.

After working on the front line of the Sars crisis, which struck terror into the region and beyond in 2003, he says the deadly pneumonia was in retrospect an "easy disease to control" because it was only spread by people who had fevers.

"Usually for influenza, it's almost impossible to control. That's why we have huge outbreaks every year."

To date, most bird flu victims have caught the disease from animals, but the fear is it will mutate into a form that can spread easily among humans, triggering a contagion that could kill tens of millions of people.

A baffling problem

Oshitani, who specialises in communicable disease surveillance and response, says WHO experts are working on models to prevent a pandemic if an outbreak of the mutated virus was detected in time, but figuring out how to contain such a virulent disease is baffling them.

"It's not simple and we don't have any experience. (With) polio control or measles control we have a lot of experience ... but for this particular case it's extremely difficult to come up with a good plan," he says.

Measures being contemplated are to treat infected people and surrounding communities with antiviral drugs, and restrict their movement to prevent others from the disease.

But if a deadly flu virus was being spread by people with no symptoms, whose movements were not able to be controlled, in a country that could not afford expensive medicines, halting an outbreak would be almost impossible, he says.

Oshitani says the WHO has a stockpile of 60,000-70,000 doses of antiviral drugs in its Manila regional office, but the logistics of getting them to a remote community in time to stop the disease would be difficult.

"Our window of opportunity will be just two or three weeks," he says. After that the virus would almost certainly have moved on.

And in teeming cities like the Thai capital Bangkok or Vietnam's Ho Chi Minh City, the sheer mass and movement of people would mean the virus would have to be allowed to run its course through the population.

"There are no other alternative tools to stop the spread," Oshitani said.

What would happen then is the stuff of nightmares.

Total chaos

Water and electricity supplies could be disrupted because utility workers are too sick to maintain them, the public transportation system could be abandoned for fear of infection, and those who cannot afford drugs would succumb in huge numbers.

"If more than 20 percent of the population is affected, it could affect a whole range of social activities," Oshitani said, adding the crisis would no longer be just a health issue, but one which could damage entire economies.

"People do not want to get infected in a bus or the train. People may not want to go to the supermarket. So how to maintain that social life is a big challenge, particularly in urban areas."

Oshitani said there were worrying signs a mutation is looming. Growing numbers of people are being infected with bird flu - 64 cases have emerged in Asia so far this year, compared with 44 for the whole of 2004.

The disease is still circulating widely in the region, and the virus has continued to change and mutate since jumping to humans in 1997, when it killed six people in Hong Kong.

Since resurfacing in 2003, it has killed 55 people in Asia including 39 in Vietnam, 12 in Thailand and four in Cambodia.

"These are all bad indicators in terms of the risk of a pandemic," he said.

Experts from the World Organisation for Animal Health and the United Nations' Food and Agriculture Organisation this week unveiled a 100 million dollar plan to rein in bird flu within a decade.

But as a backup measure, the WHO is urging countries to prepare by stockpiling antiviral drugs and readying hospitals to deal with large numbers of sick people.

While some developing countries have started to work on a strategy, others with less resources such as Cambodia and Laos have barely begun, Oshitani said.

"If you start thinking of what you should do after a pandemic starts, it will be too late," he warned.