Nguyen Sy Tuan can barely talk. His wasted frame is tucked beneath a thin white sheet on the hospital cot. His cheeks are sunken and his bulging eyes stare blankly at the ceiling.

But the young man has begun to eat rice again and can finally breathe without a mechanical ventilator, a dramatic turnaround for a bird flu patient whom doctors had assumed would die.

More than a year after avian influenza emerged in East Asia, killing more than two-thirds of those confimed to have the virus, Vietnamese doctors are reporting that the mortality rate in their country has dropped substantially.

But while this is good news for survivors, it could mean the outbreak of bird flu in Southeast Asia is taking an ominous turn. If a disease quickly kills almost everyone it infects, it has little chance of spreading very far, according to international health experts. The less lethal bird flu becomes, they say, the more likely it is to develop into the global pandemic they fear, potentially killing tens of millions of people.

``The virus could be adapting to humans,´´ said Peter Horby, an epidemiologist with the World Health Organization in Hanoi. ``There´s a number of indications it could be moving toward a more dangerous virus.´´

The mortality rate for bird flu in Vietnam this year is about 35 percent, almost exactly half that of last year, according to Health Ministry statistics. The mortality rate of the 1918 Spanish flu pandemic, by comparison, was less than 5 percent, but the outbreak killed an estimated 40 million people worldwide.

Officials said the drop in the bird flu mortality rate was more marked in northern Vietnam than in the south. While the virus in southern Vietnam is still killing at the same pace as last year, the rate in the area around Hanoi and elsewhere in the north has dropped from that level to as low as 20 percent.

Vietnamese health experts said their suspicion that the disease is shifting is further supported by preliminary research showing a genetic change in the virus in the north resulting in the production of a protein with one less amino acid than in the south.

Health researchers believe that nearly all the 52 people known to have died of bird flu in Southeast Asia caught the virus from infected poultry.

But with more cases among families reported in Vietnam this year - including that of Tuan, his sister and their grandfather - experts say they are growing increasingly suspicious that the disease has begun passing from one human to another.

Also worrying is the discovery of at least five cases, including that of Tuan´s grandfather, in which people tested positive for bird flu but showed no symptoms. This could make it more difficult to contain an epidemic because people could transmit the disease without anyone realizing it.

Last year, US researchers reported that ducks in Southeast Asia had begun carrying the bird flu virus without showing symptoms. Now, scientists in Vietnam have found numerous asymptomatic cases in the country´s vast chicken population, according to Nguyen Tran Hien, director of the National Institute of Hygiene and Epidemiology.

``It seems that the virus may adapt in humans and in poultry a little bit. Therefore, the symptoms are not as severe as before,´´ Hien said. ``Also, the transmission may be faster and easier.´´

Moreover, the existing virus strain is not the only threat. Each human case also presents a chance for the virus to swap genetic material with an ordinary flu bug - if the person becomes infected with both strains at the same time - potentially creating a hybrid that is lethal and even easier to catch.

``We are concerned that if the virus is changing, maybe a new virus is coming in the future,´´ Hien said.

Vietnamese and international health officials say they are confident that the mortality rate has dropped but are not sure by how much. Better screening and wider public awareness of bird flu could mean health workers are catching and recovering from milder cases that would have gone unreported a year ago.

WHO officials have complained that Vietnam is reluctant to provide detailed information about human cases. Senior health ministry officials respond that reports are provided in accord with national regulations.

The question now is whether bird flu in Vietnam has begun passing among humans. If it has, Nguyen Duc Tinh, a nurse who treated Tuan at the Thai Thuy district health center and fell sick with bird flu soon after, would be a likely case. Tinh, 26, said he had no contact with poultry for a month beforehand despite government accounts attributing his illness to infected chickens.

Tinh said he was the hospital staff member who had the closest contact with Tuan during his brief stay at the health center, taking his blood pressure and temperature, giving him injections and helping him walk. Within a week, Tinh had developed muscle aches and a fever, symptoms of what he believed was a common flu. But when the fever subsided and then returned two days later, he grew alarmed.

``Then I suspected I had bird flu,´´ he recalled, his brown eyes widening. ``I was really, really afraid of dying.´´

But just two weeks after joining Tuan in the Hanoi hospital, Tinh was discharged and went back to his village.

``I had lost hope when the fever came a second time,´´ he said. ``When I returned to my home town, I felt as if I were born again.´´