THURSDAY, Oct. 4 (HealthDay News) -- U.S. scientists say they´vespotted a crucial step the deadly H5N1 bird flu virus must take in orderto spread easily in humans.

Since H5N1 first appeared in 1997, there have been more than 250 humaninfections. Of those, 150 were fatal. Most of the human infections werethe result of close contact with infected birds. So far, the virus has notdeveloped the ability to spread easily among humans.

Now, researchers led by University of Wisconsin-Madison virologistYoshihiro Kawaoka have pinpointed a single change in a viral protein thathelps H5N1 infect the cells of the upper respiratory system in mammals.The adaptation could enable the virus to infect a wider range of celltypes and spread more easily among humans, the scientists said.

Being able to establish itself in the upper respiratory system enableseasy transmission of the virus through coughing and sneezing, Kawaokanoted. However, other yet-to-be identified changes would have to occurbefore the H5N1 virus could potentially trigger a flu pandemic.

The finding appears in the journal PLoS Pathogens.

"The viruses that are in circulation now are much more mammalian-likethan the ones circulating in 1997," Kawoka said in a prepared statement."The viruses that are circulating in Africa and Europe are the onesclosest to becoming a human virus."

More information

The U.S. Centers for Disease Control and Prevention has more about birdflu.