SCIENTISTS believe they have found the answer to why bird flu does not spread easily between people.
The virus prefers to infect cells in the lungs instead of areas like the nose and windpipe.
Therefore, it is not easily coughed or sneezed out into the air, say the US researchers.
But that behaviour could change if the virus mutates.
Experts say the new research does not indicate how likely the virus is to change genetically and unleash a worldwide outbreak of lethal flu.
More than 180 people are known to have been infected with the bird flu virus H5N1. Virtually all are believed to have caught it from infected poultry.
But scientists say the virus, which is prone to mutation, could transform itself into a version that spreads easily from person to person.
That germ could touch off a pandemic.
Ordinary flu viruses spread when an infected person coughs or sneezes, blasting out tiny droplets carrying the germ.
Bird flu viruses use a specific kind of docking site to enter cells they infect. Human flu viruses use a different one.