Some 400 health and veterinary officials from 100 countries gathered in Geneva to hammer out a strategy to stop the deadly H5N1 virus which is endemic in poultry in much of Asia from triggering a human pandemic that could kill millions.
The Director-General of the World Health Organisation (WHO) Lee Jong-Wook said H5N1 avian influenza, known to have killed 63 people in four Asian countries and led to the culling of 150 million birds world-wide, was on the move.
"We have been experiencing the relentless spread of avian flu. Migratory birds, as they move around the world to seasonal breeding and feeding grounds, are infecting domestic poultry flocks around the world," Lee told the conference.
He said it was only a matter of time before an avian flu virus, most likely H5N1, acquired the ability to be transmitted from human to human.
"We don't know when this will happen, but we know it will happen," Lee said. "No society will be exempt."
The virus remains hard for people to catch and is passed on almost exclusively through human contact with birds. But should it spark a human pandemic, the cost may be huge.
In a report on the bird flu threat, the World Bank said a two percent loss of global GDP during an influenza pandemic - like that caused by Sars in East Asia during the second quarter of 2003 - would represent about $200 billion in losses in one quarter or $800 billion over a year.
But experts at the conference said there were grounds for limited optimism, if countries could work together.
"The more I find out about an influenza pandemic and the lack of readiness, the more I am concerned. But I am more hopeful about the scope for international co-operation," said David Nabarro, the senior UN co-ordinator for avian and human influenza.
The World Bank report, obtained by Reuters, said a previous study on flu pandemics had suggested that any new pandemic could cause between 100,000 and 200,000 human deaths in the United States alone, which it said translated into economic losses for the country of between $100 billion and $200 billion.
"If we extrapolate from the US to all high-income countries, there could be a present-value loss of $550 billion," the report said.
"We are broadly worried about trade in a globalised world, but two industries that would be particularly hit are tourism and agriculture," said Jim Adams, a senior World Bank official.
Experts are racing against time to prevent the spread of a virus which has recently moved from Asia to eastern Europe and is expected to move into the Middle East and Africa.
As part of the global strategy, the World Bank will launch an appeal for a $1 billion package at the conference, half of it to be provided through its grants or interest-free loans and half through a trust fund financed by donors, Adams said.
Experts told the conference the main focus should be on containing the spread of the virus among poultry and limiting contact between sick birds and humans.