The World Health Organization has warned that between 2 million and 7 million people worldwide could die in a flu pandemic if the deadly H5N1 strain of the avian flu mutates to spread easily from human to human, something health authorities across the world are working together to prevent.

"Influenza pandemics are remarkable events that can rapidly infect virtually all countries," said a September 2005 WHO report on the H5N1 virus that has killed over 60 people in Southeast Asia. "Once international spread begins, pandemics are considered unstoppable, caused as they are by a virus that spreads very rapidly by coughing or sneezing."

As more and more flocks of birds are found to have been exposed to the virus, stopping the onslaught of a deadly pandemic in humans has become a top priority for governments and international health and agriculture organizations. More than 140 million birds have died or been killed as a result of the virus, according to the U.N. Food and Agriculture Organization, and new outbreaks continue in parts of Asia and Europe.

"The people of the country ought to rest assured that we're doing everything we can: We're watching it, we're careful, we're in communications with the world," President Bush told reporters at an Oct. 4, 2005 news conference in Washington.

Until July 2005, avian influenza, also know as the bird flu, was isolated in Southeast Asia where in 1997 the first recorded outbreak in humans infected 18 people in Hong Kong and killed six. Then, in 2003 and 2004, several Asian countries -- South Korea, Vietnam, Japan, Thailand, Cambodia, Laos, Indonesia, China and Malaysia -- reported cases of the disease in poultry.

Now, authorities at the Food and Agriculture Organization fear that migratory birds traveling between continents may help spread the virus from birds in Asia to wild birds and poultry in parts of Europe, Africa and the Middle East as birds return to cooler temperatures in the spring.

"The spring migration of 2006 may result in the spread of HPAI (highly pathogenic avian influenza) H5N1 virus further across Europe since birds migrating from southern zones will have intermingled with European Russia and Siberia-origin birds during the 2005/2006 winter nesting areas," said a September 2005 FAO study.

To combat bird-to-human contact, the FAO and the WHO have said stopping the virus at its source -- farms -- could help avert a pandemic. This means some farmers, both rural and commercial, will be forced to destroy many more birds before the virus is contained.

Authorities fear that outbreaks in small, rural communities could lead to a mutation of the virus and a jump from bird-to-bird transmission and rare human infections to widespread human-to-human transmission.

In areas where people have contracted the avian flu, most of the cases have resulted from direct contact with their sick birds.

But because many small farmers are not being compensated for lost birds, few are willing to admit to having sick flocks, making fighting the spread of the virus difficult. According to the FAO, economic losses to the Asian poultry industry have reached $10 billion.

"We're still seeing in some places when there are outbreaks in the poultry, the owner of those birds they don't want to kill the birds," Hong Kong microbiologist Dr. Chen Jixiang told ABC News.

Countries also have resorted to stiffer trade practices and outright bans on products, which could mean further economic losses for farmers in affected countries. Following the discovery of bird flu in Turkey and Romania, the European Commission banned imports of live poultry and pet birds and is considering banning imports of poultry from countries where the virus has been found, such as Croatia.

International agricultural organizations are urging limited contact between workers and poultry at production facilities and increased monitoring of open air facilities where poultry could mix with wild birds.

The FAO has also called on countries to prevent further spread of the disease by adopting safety and security measures at commercial farms, including better organizing how the birds are kept, restricting the production and movement of certain poultry and providing workers involved with killing the animals protective equipment and disinfectants.

In case a human flu pandemic does occur, the United Nations is urging countries to develop medical plans by instituting influenza response plans and stocking up on supplies of antiviral drugs like Tamiflu and Relenza that can be used to treat humans with the flu virus.

To date, only a small percentage of countries have complied with WHO directives to stockpile drugs, increase response time to warnings of outbreaks and to help stop the virus at the root. Many countries, including those in Southeast Asia, have been unable to comply because of limited financial resources.

"Because these activities are costly, wealthy countries are presently the best prepared," the WHO reported. "Countries where H5N1 is endemic -- and where a pandemic virus is most likely to emerge -- lag far behind."

About 23 countries have ordered antiviral drugs, according to the organization, and only 10 countries are actively producing domestic supplies of a vaccine. "The majority of developing countries would have no access to a vaccine during the first wave of a pandemic and possibly throughout its duration," the WHO reported.

-- Compiled by Kristina Nwazota for the Online NewsHour