The disease, which was first identified in Italy more than 100 years ago, occurs worldwide. There are 15 different strains of the virus, but the current strain deadly to humans is identified as H5N1.
All birds are thought to be susceptible to the flu, though some species are more resistant to infection than others. Infection causes a wide spectrum of symptoms in birds, ranging from mild illness to a highly contagious and rapidly fatal disease.
For humans, the symptoms are similar to other types of flu -- fever, sore throat and cough. Not all cases are fatal.
Following the 1997 outbreak, Hong Kong implemented a number of preventive actions, including human and poultry surveillance programs, and stockpiling antiviral drugs for treatment. The WHO stresses the importance of sharing information openly to streamline preventive strategies.
"Eradication of the virus from the eight affected Asian countries will not be easily achieved," said Domenech. In Vietnam's case, he even suggested massive vaccination as the only way to reduce infection.
Two top science journals, Science and Nature, reported that the deadly H5N1 flu virus has spread among migratory geese in China. Previously, the occasional wild bird had been found dead near an infected poultry farm, but this was the first time scientists have seen the virus spread among wild birds. The migratory nature of these birds leaves open the possibility that they could spread the virus. Genetic analysis of the virus shows that it is similar to the virus that caused human illness in Thailand and Vietnam.
Popping pills to ward off infection won't do the trick, either.
"The use of an antiviral drug in poultry will create drug resistance and will hamper the treatment of avian flu in humans," Domenech said. All type A influenza viruses, like the bird flu H5N1, are genetically adaptable and readily mutate, according to WHO.
Its adaptability makes it all the more threatening. Experts fear it could evolve into a human virus, making it all the more deadly. At least two people have contracted the virus from another person.
Based on historical patterns, the WHO expects influenza pandemics to occur, on average, three to four times each century when new virus subtypes emerge and are readily transmitted from person to person. In the 20th century, the great influenza pandemic of 1918-19, which caused an estimated 40 million to 50 million deaths worldwide, was followed by pandemics in 1957 and in 1968.
Experts agree that another influenza pandemic is inevitable and possibly imminent.
Most influenza experts believe that the prompt culling of Hong Kong's entire poultry population in 1997 probably averted a pandemic.
The CDC predicts that a "medium-level epidemic" could kill up to 207,000 Americans, hospitalize 734,000, and sicken about a third of the U.S. population. Direct medical costs would top $166 billion, not including the costs of vaccination. An H5N1 avian influenza that is transmittable from human to human could be even more devastating, assuming a mortality rate of 20 percent and 80 million illnesses.
Right now, the FAO's main focus remains the more affected countries. It estimates it will take more than 10 years for China and Indonesia to get rid of the virus, while Vietnam could wipe it out in six years.
The U.N. agencies want further transparency and better surveillance to fight the disease. In addition, they would like to set up a $102 million fund to help countries for the next three years. More than half of the money will be spent in Vietnam, Indonesia, Cambodia and Laos.