HONG KONG (AP) -- Bird flu's rapidly changing genetic makeup and the cramped quarters of Asian chicken farms and markets are likely behind the disease's explosive spread, a World Health Organization expert said Friday.

The H5N1 virus has swept through South Korea, Japan, Vietnam, Thailand and Cambodia, infecting millions of chickens. In Vietnam, the virus has killed at least five people. Two boys are sick with the disease in Thailand, and a man suspected of having the disease died late Friday.

Scientists have reached no firm conclusions on why it is so contagious, but a leading theory is its adaptability, said Robert Webster, a U.S.-based virologist who researches animal viruses for WHO.

Once birds or humans develop antibodies against a particular incarnation of H5N1, "the virus has to change to escape that immunity ... that's what it does," he said.

"It's constantly changing," Webster said in Hong Kong, where he is conducting research at a local university.

WHO says it has found "significant differences" between H5N1 strains taken from humans and chickens in this year's outbreak and strains from earlier outbreaks of bird flu in Hong Kong in 1997 and 2003, indicating the virus has mutated.

Another likely factor is the high density of chickens at large farms and traditional live poultry markets -- where birds are kept in small cages and in close proximity, Webster said.

Such settings could encourage a process called "reassortment" in which the H5N1 virus might combine with human flu viruses, he said.

Bird flu viruses that have undergone this process can move directly from human to human, instead of from bird to human. No cases of human-to-human transmission have been reported yet in the current epidemic.

Because of the high death rate among humans infected with H5N1, such a reassortment "is one of the major fears of infectious disease experts throughout the world," the British medical journal The Lancet said in an editorial about bird flu in its current issue.

"The prospect of a worldwide pandemic is massively frightening," it said in the editorial.

Reassortment leading to the spread of bird flu among people most recently occurred in 1957 and 1958, Webster said.

In 1957, duck flu merged with a strain of human flu in southern China, crossing over to humans and resulting in human-to-human transmission, he said. An outbreak in Hong Kong the next year evolved similarly.

Webster said those precedents are important in understanding the current outbreak -- although there's no hard evidence yet suggesting that the same phenomenon is unfolding this time.

It's also possible that migratory birds are carrying H5N1 from country to country, but this has not been proven either, Webster added. That would help explain why the disease has appeared so suddenly in so many places.

The possibility was first raised when a group of water fowl in a Hong Kong park died of H5N1 in 2002 -- the first time migratory and aquatic birds naturally acquired the virus, he said.

The Lancet said the H5N1 outbreak shows the need for a broader approach to disease control.

"One thing is clear: given that all new infectious diseases of human beings to emerge in the past 20 years have had an animal source, veterinary science and animal husbandry are as important for disease control as clinical medicine," it said.

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