The H5N1 strain of bird flu virus has travelled across Europe, Africa and Asia and has infected 183 people since it re-emerged in 2003, killing more than 100 people worldwide.
Flu viruses ae known to target people attach to cells up in the airway. This maxmises the chance of it being passed on by coughing and sneezing. Researchers have found that the bird flu virus attaches itself to cells, deep in human airways. However, it cannot be passed on from human to human.
If the virus gains the ability to mutate, scientists fear it could result in human flu pandemic with millions of death worldwide.
The researches from the university of Wisconsin has investigated why the deadly virus cannot spread between humans enough though it can replicate itself in human lungs.
The molecule targeted by bird flu viruses can be found on cells in side the lungs, therefore the virus is found buriedso deep in the lungs that it was very unlikely for it to be transmitted by cough and sneezes.
If the virus had the ability to infect the cells around the airways researchers believe it could cause a human pandemic.