Scientists have learned why the deadly H5N1 bird flu does not easily infect people and spread between them.  They have discovered a biological barrier that has so far prevented a feared human pandemic.

Avian influenza has infected nearly 200 people worldwide and killed about half of them. Each documented case has been the result of close contact with infected birds. But the virus does not easily jump from birds to humans and there are no known cases of spread between humans.

The answer to this biological mystery is now revealed by Dutch and Japanese scientists working independently of each other and reporting respectively in the journals "Science" and "Nature.". They say that the H5N1 virus attaches to and enters cells too deep down in human lungs to spread easily by coughing and sneezing. The leader of the Dutch research team, Thijs Kuiken of Erasmus Medical Center in Rotterdam, says this contrasts with human influenza strains, which attach to cells in the nose and throat and are easily transmitted from person to person.

"Being able to attach higher up in the respiratory tract may mean, we're not sure, but it may mean that it comes more easy for such a person to become infected in the first place, and in the second place to transmit the virus to another person," Kuiken says.

Kuiken's team and one led by University of Wisconsin virus expert Yoshihiro Kawaoka with colleagues in Tokyo made their discovery by studying tissue removed from various parts of the human respiratory tract.

In addition to explaining why people do not spread H5N1 bird flu, their finding explains why transmission is relatively rare from birds to humans.  The H5N1 virus enters birds in a different part of the respiratory tract.  A protein on the viral surface recognizes binding points in birds' upper respiratory system, or windpipe, locations it does not recognize in the human breathing passageway.

The Dutch scientist says the research also tells us why the death rate is so high, about 50 percent, among people who have been infected with the H5N1 avian flu virus.

"They explain to us why in the few cases where humans have been infected with the H5N1 virus, the main lesion is seen in the lungs, and often people that have the H5N1 virus get a severe pneumonia and can even die from it," Kuiken says.

Kuiken says that for the H5N1 virus to infect people in a higher part of the breathing passage and ease transmissibility, it would have to mutate. The new studies do not indicate how likely such a mutation would be.

He and the Japanese scientists point out that by using their laboratory techniques, researchers can monitor viruses isolated from humans infected with bird flu to determine if the viral binding protein is indeed changing in a way that would threaten a human pandemic.