Avian influenza is making the world a global village -- or, more precisely, a global barnyard -- in a way that demands international cooperation to a degree not seen previously on a health issue, experts said Monday as 600 people from 100 countries began a conference on how to prevent bird flu from becoming a human pandemic.

Wealthy countries will have to provide hundreds of millions of dollars for virus surveillance and testing, vaccine production and antiviral stockpiling, many delegates said. Developing countries, particularly in Southeast Asia, where the H5N1 bird flu virus has circulated since 1997, must create compensation programs so that farmers won't conceal outbreaks in their flocks.

In addition, several experts called for changes in the traditional ways poultry is raised and marketed in the developing world in order to put more distance between birds and their keepers. Scientists will also need to learn a lot more about the ecology of flu viruses in migrating wild birds, which apparently recently carried H5N1 from East Asia to Europe.

"We need to deal with this together. . . . If one country is inadequately prepared, it is a threat to every other country," said Bernard Vallat, head of the World Organization for Animal Health, an international agency known by the French acronym OIE.

OIE is sponsoring the meeting with the World Health Organization (WHO), the U.N. Food and Agriculture Organization (FAO) and the World Bank. It is being held at WHO's headquarters in Geneva.

The potential effects of an influenza pandemic are enormous. WHO estimates that a pandemic comparable to the mild Hong Kong flu of 1968 could kill as many as 7.4 million people. If it were as lethal as the 1918 Spanish flu virus, which killed 50 million, the toll would be much, much higher.

In 2003, a short-lived and well-controlled outbreak of SARS, or severe acute respiratory syndrome, caused a 2 percent drop in Southeast Asia's gross domestic product in a single quarter. A pandemic that lasted a year, as most do, would produce economic losses of $800 billion, said Milan Brahmbhatt, an economist with the World Bank.

The latest outbreak of the H5N1 strain, which began in December 2003, has cost Southeast Asia more than $10 billion and depressed its GDP by 1.5 percent. Vietnam and Thailand have each lost about 15 to 20 percent of their poultry stocks from death and intentional culling of infected flocks.

"The benefits of preventing or mitigating an outbreak are likely to be very high," Brahmbhatt told the delegates gathered in the round, wood-paneled assembly room at WHO's headquarters.

Virtually everyone agreed that, despite the 124 human cases and 63 deaths from the H5N1 strain since December 2003, the virus remains overwhelmingly an animal pathogen. However, the more animals that contract it, the more chances it has of developing mutations that might allow it to infect people more easily than it does now.

Large numbers of dead or dying birds also mean that more people will be exposed to the H5N1 virus, which could theoretically "reassort" with a human flu virus in an infected person, forming a hybrid with new characteristics.

"The control is at the level of the animal. The window of opportunity for doing that is still open. The virus has not yet reassorted or mutated," said Samuel Jutzi, an FAO official.

A delegate passes an image meant to keep birds from hitting windows at WHO headquarters in Geneva.