“We have identified a specific change that could make bird flu grow in the upper respiratory tract of humans,” said lead researcher Yoshihiro Kawaoka of the University of Wisconsin-Madison.

The latest mutation gives the virus the ability to thrive at much lower body tissue temperatures. Birds usually have a body temperature of 106 degrees Fahrenheit while humans are typically at 98.6 degrees.

The human nose and throat, where flu viruses usually enter, is typically around 91.4 degrees. This kept earlier versions of the virus from being able to infect those areas. This new mutation allows H5N1 to live well in the cooler temperatures of the human upper respiratory tract, according to Kawaoka.

He said that recent samples of the avian influenza virus taken from birds in Africa and Europe all contained the mutation. The H5N1 strain, which has killed or caused the slaughter of millions of birds, has infected 329 people in 12 countries since 2003, killing 201 of them.

It has only rarely been passed from human to human, but the wrong combination of mutations could give it the ability to spread into a global epidemic.

Map graphic: World Health Organization