Bird flu has infected people in at least two countries and the World Health Organization says it could already be entrenched in poultry populations across Southeast Asia. The more contact humans and pigs have with the virus, the greater the chance of it changing and spreading between humans.

Health experts in Thailand and Vietnam have confirmed human cases of bird flu and massive outbreaks of the disease at farms.

Scientists say the human victims likely caught the disease from infected chickens or dried chicken waste.

Bob Dietz, a spokesman for the World Health Organization, says health experts are only now beginning to understand just how widespread the disease is across Asia.

"The H5N1 virus, the strain that we are seeing recently in Vietnam, is becoming entrenched in Southeast Asia," he said.

The virus has been found in recent weeks in Japan, South Korea, and Taiwan. Governments in affected areas have ordered mass culls of chickens in an effort to stop the outbreak.

Mr. Dietz says the United Nations health agency recommends culling contaminated flocks.

"Now we're encouraging getting rid of the reservoir because of the threat it poses to human beings, not just because of the threat it poses to other poultry," he stressed.

The H5N1 virus, which is lethal for chickens, appears also to be particularly dangerous to humans, especially children. Scientists are worried that the bird-flu virus could learn to spread as rapidly in humans as it does in chickens.

Microbiologist Malik Pieris at Hong Kong University explains that avian influenza viruses are prone to change. He says they can become more virulent or milder when they infect people by mutating or combining with other human influenzas.

"The problem is that the longer you allow human exposure and human infection, the greater is the risks that this virus might adapt to humans or this virus might mix its genes with a human virus," he said.

Dr. Piersis has been tracking changes in the H5N1 virus since it emerged in 1997 in Hong Kong and crossed from chickens to people. Back then, 18 Hong Kong residents became ill with the virus and six of the victims died.

According to Dr. Pieris, there is evidence that the virus has changed slightly over the years.

"From the studies we have been doing from 1999 onward, we certainly could see that from about the year 2000, the H5N1 viruses started to mix their genes with a whole range of other avian influenzas," he said. "So this H5N1 virus has become very promiscuous in mixing and matching its genes."

One Hong Kong biotechnology firm, DNA Chips, produces test kits to check chickens and humans for the bird flu.

A company researcher, Dr. Natalie Dillon, says so far there is no evidence of significant changes in the virus. "The transmission from birds to humans is very difficult - so that's the reason for the low number of cases," she said.

Dr. Dillon says, however, the presence of the disease in farms across Southeast Asia is dangerous because it means other animals are at risk of infection.

Dr. Dillon says pigs carry human influenzas and they could become a mixing place for the H5N1. This potentially could give rise to new bird flu viruses capable of causing a pandemic in people.

Scientists says that flu pandemics that began in Asia in the 1950s and 1960s contained both human and avian influenza genes. Hundreds of thousands of people worldwide died in those outbreaks.