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HANOI, Vietnam -- One of the many mysteries of bird flu is that it has not infected more people like Ha Thi Quynh.

The woman in her late 30s holds up a plump, live goose by its feet at Hanoi's largest poultry market. Although blood, feathers and bird droppings cling to her pants and rubber sandals, she doesn't worry about bird flu.

"I have no problem," she says. Quynh has driven a motorbike loaded with about 35 chickens and geese on a two-hour trip to the market every day for the past 10 years. "If customers ask me to slaughter the chicken, then I will do it."

Quynh and the others at Long Bien market say they're living proof bird flu is hard for people to catch. They work without fear or protective gear in a place where fresh blood runs through open gutters and stray feathers glide through the humid air, thick with the stench of death. They say not a single person from the market has ever gotten sick or died from the H5N1 bird flu virus.

Researchers agree. They're just not sure why these people have stayed healthy.

Farmers at large poultry facilities and those who transport, sell and slaughter birds daily typically have not been infected since the virus began spreading through Asia in late 2003.

Why has the disease attacked mostly healthy children and young adults, who may have had a few chickens pecking in their back yards or villages? Is there some sort of immunity acquired by commercial farmers and others who have worked around the poultry for so long, or is it some other reason?

VICTIMS WERE STARTLINGLY YOUNG

"The honest truth is that on a lot of answers to these questions, your guess is almost as good as mine," said Dr. Jeremy Farrar, director of Oxford University's clinical research unit at the Hospital for Tropical Diseases in Ho Chi Minh City.

Farrar was part of a large research committee that reported in the New England Journal of Medicine last month what's known about those infected with bird flu. The youth of most of the victims was striking.

Out of 41 confirmed cases examined in the article (which doesn't include all of them), the ages of those infected ranged from 2 to 58. In Thailand and Cambodia, researchers calculated the median age of those infected: age 14 in Thailand and 22 in Cambodia. For the Vietnam outbreak in 2004, the average age was 14.

The researchers also noted that recent infections have caused "high rates of death among infants and young children. The case fatality rate was 89 percent among those younger than 15 years of age in Thailand."

A MATTER OF IMMUNITY -- OR MATH?

As migratory birds spread the virus from Asia into Russia, Turkey and Romania, many more people are asking how and why people become infected. World health experts warn the virus could mutate into a form that's easily passed from person to person, possibly sparking a global outbreak that kills millions.

But for now, at least 121 people have been infected and at least 62 have died in Southeast Asia, mostly in Vietnam.

Farrar said it's possible some people could have a pre-existing immunity protecting them, but there has been no research to prove that. He also said the type of contact between people and poultry could play a role.

For example, a man who grooms and bandages a prize-winning fighting cock or a child who plays with a pet duck likely have very different relationships with birds than mass poultry farmers and those who slaughter the birds.

It could also just boil down to math, said Peter Horby, an epidemiologist at the World Health Organization in Hanoi.

"Maybe it's just the actual numbers. Eighty percent of the population is living with chickens . . . compared to a relatively low number of people being exposed to a high risk," he said. "The risk is actually very low, but because there are so many more people daily exposed to chickens, that's where most of the cases are occurring."

He and other researchers have theorized that more children may be at risk because they're closer to the ground, crawling or walking barefoot across earth sprinkled with poultry droppings, or putting soiled hands or dirt into their mouths.

CAREFUL MONITORING REQUIRED

But most of it is just guesswork.

Dr. Frederick G. Hayden is a virus expert from the University of Virginia, said scientists need to better understand how clusters of family members got sick, how and where the virus enters the body and why it sometimes spreads from the lungs to other organs and the nervous system.

"Clearly, this is a virus that is incredibly pathogenic in multiple species and so it really requires careful monitoring," Hayden said from Ho Chi Minh City, where he was working with health officials from Vietnam, Thailand and Indonesia.

Hayden said he believes a flu pandemic is coming, but there's no way of knowing when.

"My sincere hope is that the virus will give us time," he said. "This is a virus that is a global threat. There's no question about it, so the questions that can be answered here will have implications on an international scope."