Scientists believe they may have discovered a reason why the deadly H5N1 bird flu virus cannot yet jump easily between humans.

Flu viruses which target man tend to attach to cells further up the airway - maximising their chances of being passed on by coughing or sneezing.

Researchers found the bird flu virus attached itself to cells deep down in the human airways.

The University of Wisconsin research is published in the journal Nature.

The H5N1 strain of bird flu has spread across Europe, Africa and parts of Asia, has killed more than 100 people worldwide and infected about 180 since it re-emerged in 2003.

But it still cannot jump easily from human to human.

Scientists fear that if it gains that ability and mutates it could result in a human flu pandemic, with millions of deaths world-wide.

Target molecule

The Wisconsin team investigated why the virus could not spread easily between humans despite the fact that it could replicate efficiently in human lungs.

Flu viruses infecting humans and birds are known to home in on slightly different versions of the same molecule, found on the surface of cells which line the respiratory tract.

The latest study found the version of the molecule targeted by human viruses was more prevalent on cells higher up in the airway.

The molecule targeted by bird viruses, on the other hand, tended to be found on cells deep within the lungs, in structures called alveoli.

Thus the bird flu virus tended to be buried so deep in the lungs that was unlikely to be spread by coughing or sneezing.

If the virus was to acquire the ability to infect cells higher up in the airway, it could take a crucial step towards causing a human pandemic, researchers believe.

Victims 'unlucky'

Professor Ian Jones, of the University of Reading, said the study provided some explanation of why people, particularly children, had caught the virus and died and yet it had remained "bird flu".

"It seems they were just really unlucky and transmitted enough virus to their mouths for it to gain access to the lower lung, a distance shorter in children than adults.

"Casual contact with the virus may therefore not be as dangerous as initially thought."

However, Professor Jones said it was possible that the virus could mutate to gain the potential to attach to cells in the upper airway.

"It remains the case therefore that planning for that eventuality makes a great deal of sense."

Dr Laurence Tiley, a lecturer in molecular virology at the University of Cambridge, said: "This may at least in part explain why H5N1 is inefficient at transmitting person to person, although I doubt that it is the complete answer."