When bird flu infects people, the virus is more concentrated in the throat than the nose, the opposite of human flu. This finding may help doctors more quickly diagnose the bird flu in people.

The disease has been linked to the deaths of more than 140 people worldwide, mostly among Asian farm families who live in close contact to birds. There have been no reports of infections of people in the United States.

Health officials have monitored the disease as it moves through poultry and other animals. The fear is it could mutate into a form that spreads easily from person to person, a development that officials say could lead to a global pandemic.

Researchers are studying the disease in an effort to find a way to prevent or block it and to treat victims.

Menno de Jong of the Oxford University Clinical Research Unit at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, reports in Monday's issue of Nature Medicine that people with bird flu had much higher levels of the virus in their throat than in their nose.

That is important in showing doctors a better way to diagnose the disease, he said. It is also important that physicians can detect the virus in diarrhea and other rectal secretions. This is one more way the disease can spread and shows the need for infection-control measures, de Jong said in an interview via e-mail.

"Our observations suggest that early recognition and early treatment may provide the best benefit. Early recognition and diagnosis will pose a challenge for clinicians," he said.

De Jong and his co-authors studied 18 people infected with bird flu, which is known as H5N1, and compared them with eight people who had common human flu viruses.

"Our observations suggest that H5N1 virus replicates to very high levels — higher than common human flu — in the respiratory system and that these high levels of virus ignite an overwhelming intense inflammatory response," he said.

In inflammation, the body's immune system causes blood vessels to allow chemicals and blood cells to leak into an infected area, designed to attack the infection, but an over-response can cause harm.

"Extensive damage to the lungs and possibly other organs are likely caused by both the direct effects of the virus as well as by the intense inflammatory response to the virus by the infected individual," de Jong said.

He said the researchers could detect the bird flu virus in the blood of people who died of the disease, but not in the blood of these who survived an infection.

"The virus in the bloodstream most likely is picked up during passage of the blood through the lungs where most virus replication occurs," he said.

"The presence of virus in the bloodstream may be a direct consequence of high levels of virus in the most important site of infection (lungs) and reflect an overall high 'bodyburden' of virus in fatal cases," he said.

Dr. Wilbur H. Chen of the University of Maryland School of Medicine in Baltimore said that researchers are clamoring for more details on how the bird flu affects humans, and in particular better ways to quickly diagnose the illness.

Chen, who was not part of de Jong's research team, said it sounds like the researchers found some useful information.

De Jong's work was funded by the Wellcome Trust.