Here are some facts about H5N1 avian flu:

-- There are dozens of known flu strains, named for two proteins each virus carries. H5N1 refers to an avian flu strain that emerged in Hong Kong in 1997, killing or forcing the destruction of 1.5 million chickens, ducks and geese, infecting 18 people and killing six. The World Health Organization says the quick slaughter of all potentially infected birds may have averted a pandemic.

-- H5N1 avian influenza re-emerged in Korea in 2003. It has now been found in birds in Cambodia, China, Indonesia, Japan, Kazakhstan, Korea, Laos, Malaysia, Mongolia, Philippines, Russia, Taiwan, Thailand and Vietnam.

-- H5N1 is considered the biggest direct disease threat to humanity because it mutates rapidly and can also acquire genes from other viruses, making it a potential human pathogen.

-- Experts estimate that if it acquires the ability to infect people easily and spread from person to person efficiently, it will make more than 25 million people seriously ill and will kill as many as 7 million.

-- Other models show the virus would make 50 percent of people where it is circulating ill, and 5 percent could die.

-- All influenza viruses change quickly, which is why the standard flu vaccine must be changed every year. H5N1 is particularly good at changing. Experts fear it may acquire a key gene from a flu virus that already easily infects humans and become a highly contagious and deadly strain.

-- Birds that survive infection with H5N1 excrete the virus for at least 10 days, orally and in faeces, making it highly likely to spread. Migratory birds, usually wild ducks, are the natural "reservoir" of avian influenza viruses, and usually do not become sick when infected. Domestic poultry, including chickens and turkeys, die quickly when infected.

-- Several companies are working on an H5N1 vaccine, and the furthest along in development is France's Sanofi-Aventis. US-based Chiron Corp. aims to test its H5N1 vaccine later this year and Britain's GlaxoSmithKline Plc plans large-scale clinical trials in 2006. The ordinary flu vaccine does not protect against avian flu.