Scientists have reported the first case of avian flu passing from person to person and causing severe disease, fulfilling what some say could be an intermediate step toward the deadly flu becoming a wider human epidemic.

The case occurred last fall in Thailand, where an 11-year- old girl who caught the virus from infected chickens apparently transmitted the disease to her mother and an aunt who had been caring for her. The girl and her mother died, while the aunt survived a severe bout of the illness.

Though investigators caution that they have found no evidence that the disease spread in a human chain to other members of the family or community, they say the cases demonstrate that the flu may be developing the ability to threaten populations.

"What this tells us, clearly, is that we are likely to see additional fatal human cases and additional clusters because the [avian flu] virus remains actively circulating in Asia," said Nancy J. Cox, chief of the influenza branch at the U.S. Centers for Disease Control and Prevention in Atlanta.

Cox and others following the developments said the findings demonstrate the need for better safeguards to contain the virus should it break out into a human epidemic.

"This really has to be an international effort," she said. "We have to improve surveillance in the countries that are being affected, so cases can be identified early and monitored for chains of transmission."

When cases are identified, public health authorities can isolate the sick and encourage family members and health care workers to take precautions such as wearing gloves, gowns and face masks.

The report by Thai health authorities and the CDC is due to appear this week in the New England Journal of Medicine. The magazine's editors made the report available to the public today.

In an editorial accompanying the article, Klaus Stohr of the World Health Organization's global influenza program said more information about the virus and its behavior is needed to aid researchers trying to develop a preventive vaccine as well as drugs to curb symptoms among those already sick.

Since the first human cases of "bird flu" surfaced in Southeast Asia in 1997, scientists have voiced concerns that the strain could trigger a worldwide epidemic, known as a pandemic. So far, outbreaks among humans have been small. From the beginning of flu season early in 2004 through November, there were 44 documented cases. Thirty-two of the victims died.

Pandemics tend to arise every generation or so, but what public health authorities most fear is one as serious as the Spanish flu of 1918 and 1919 which killed between 20 million and 100 million people worldwide - more than died in World War I.

Avian flu, so named because it travels among chickens and other birds, is particularly worrisome because of its high lethality. Scientists believe that it kills about three quarters of the people it infects, in contrast to common human strains which have mortality rates of well under 1 percent.

Scientists have been somewhat heartened by the fact that while the strain can be transmitted from infected chickens to people who handle them, it did not seem to pass with any regularity from person to person.

As long as the virus passes from birds to people - and not through human populations - it is likely to remain a fairly confined illness.

Doctors did identify a few cases of suspected person-to-person transmission in recent years, including that of a Hong Kong health care worker who developed mild symptoms after taking care of an infected patient.

"The difference here is that severe illness and death resulted after transmission from the daughter to the mother," said Cox, adding that the girl's aunt most likely caught the disease from her as well.

For a wider epidemic to erupt, the virus would have to acquire genetic features that would make it more contagious. So far, said Cox, the disease appears difficult to transmit. Otherwise, doctors would have seen widespread disease among health care workers and families and neighbors of the sick.

But some experts say the case should erase any complacency toward avian flu.

"I think people were delusionally hoping that it would never be transmitted from person to person and that would save us," said Dr. Richard Chaisson, a specialist in infectious diseases at the Johns Hopkins School of Medicine.

"It is not at this point in a form that is easily transmitted. That's the step between a small local outbreak like this and an epidemic which can then turn into a pandemic."

The girl, who lived in a Thai village with her aunt, went to a health clinic Sept. 2 last year with a fever, cough and sore throat. She was admitted to a hospital five days later, and after another day, to a larger hospital when she developed respiratory distress and went into shock. There, she was placed on a respirator but died three hours after being admitted.

Her body was cremated before investigators could study specimens that could lead to a firm diagnosis. But doctors said her symptoms almost certainly indicated she had avian flu. Also, the disease had sickened a number of chickens that ranged freely in the yard. While the girl did not handle them, she slept under the raised house in an area where the chickens roamed.

The girl's 26-year-old mother lived a distance away in Bangkok, but traveled to her daughter's bedside to take care of her. She held her closely, wiped secretions from her face and was likely exposed to her daughter's coughs. She returned to Bangkok after her daughter died. She, too, developed symptoms and died in a district hospital.

Investigators concluded that she probably caught the virus from her daughter because she was not exposed to any of the chickens, but had been exposed for several days to the ailing girl.

Investigators said it was unlikely that the aunt, who was 32, contracted the virus from the chickens because she developed her first symptoms 17 days after the last sick bird was destroyed. This is more than a week longer than the accepted incubation period.

She probably picked up the illness from her niece, whom she had cared for from the time she got sick through the first day of her hospitalization, investigators said.

Cox said there is no way to know how long it might take for the virus to pass more efficiently from person to person, if it ever does. Flu viruses mutate all the time, and a random mutation could give it the ability to race through families and communities.

Also, someone who simultaneously harbors avian flu and a more common flu virus could become an incubator for a nastier avian strain. In this scenario, the two viruses exchange pieces of genetic information. In the swap, bird flu picks up the very genes that make common flu so contagious among humans.

"That is the nightmare scenario," Cox said.