The H5N1 virus responsible for the current virulent strain of bird flu has evolved into two genetically distinct strains, US scientists have confirmed.
They fear this could increase the risk to humans - and complicate the search for an effective vaccine.
The US team analysed more than 300 H5N1 samples taken from infected birds and people between 2003 and summer 2005.
Details were presented to the International Conference on Emerging Infectious Diseases in Atlanta.
Full story at BBC.