The bird flu that has spread through chickens and killed at least 20 people in Asia has recurred in Thailand in places were it had previously been contained. Thailand's Deputy Agriculture Minister Newin Chidchop says new outbreaks of the bird flu have occurred in nine Thai provinces.

In eight of those provinces, the government had thought the disease was contained, after it had culled thousands of chickens.

The World Health Organization says that infections in humans are rare. Despite the millions of cases in chickens, bird flu has crossed into humans only about 30 times this year - all of them in Thailand or Vietnam. But at least 20 of the victims have died.

Many experts say more human cases may be found in impoverished areas of Indonesia and China, where health care systems are too limited to be able to track outbreaks.

The WHO says it appears that the human victims caught the illness directly from infected chickens, as the virus does not spread easily between people. Bird flu first sickened 18 people in Hong Kong in 1997, killing six of the patients.

After almost a week of no new human cases, Vietnam confirmed that a boy hospitalized on February 10 has the virus. Many children are among the human victims of the bird flu in Thailand and Vietnam.

Maria Cheng is a World Health Organization spokeswoman based in Hanoi, and spoke recently about the risk for children.

"We still do not know why children had been more affected [than] adults. ? There is some speculation that children may have had more exposure to sick poultry than perhaps adults," she said.

Ms. Cheng added that the threat to humans would continue as long as the disease exists in poultry flocks in Asia. She said recurring outbreaks on farms could take as long as two years to stamp out.

Tests on a leopard from a Thai zoo confirmed the presence of the bird flu virus, but it is not clear how many other species may be at risk of infection. Experts say many waterfowl carry the virus without showing symptoms.

At least 10 Asian countries have confirmed outbreaks in poultry, although some of them have a milder type of the disease. Outbreaks of the milder virus also have been found in some parts of the United States.