The World Bank and the United Nations officials warned the possible occurrence of pandemic of avian influenza on Sunday.

"There will be a world animal and human influenza pandemic one day, but we don't know when," said David Nabarro, Senior United Nations System Coordinator for Avian and Human Influenza, at a press conference held here.

According to World Bank estimate, a severe flu pandemic among humans could cost the global economy up to about 3.1 to 4.8 percent of gross domestic product (GDP), which is between 1.25 trillion and 2 trillion U.S. dollars of a world GDP.

"Over the last 12 months, the avian influenza has gone global, spreading rapidly beyond its East Asian stronghold to the countries in South Asia, Europe, Middle East and Africa. So far, more than 50 countries have reported H5N1 outbreaks, most of them since January 2006, causing an estimate of 220 million bird deaths and significant damage to rural livelihoods, especially in the poorest areas," according to a World Bank's press release.

Nabarro said that they are now concerned that the H5N1 avian influenza virus would mutate to a new kind of virus and the virus would be transmitted from human to human someday.

"The world community has stood up and worked together to fight against the virus," said Jim Adams, Head of the World Bank's Avian Flu Taskforce.

The World Bank is ready to support developing countries to fight avian influenza in animals, while simultaneously preparing for a possible human flu pandemic through a global funding program, he added.

Source: Xinhua