There has been a mutation allowing the bird flue virus to breed effectively on mammal tissue and become highly virulent.
The Vietnamese research has shown the bug is adapting to infect humans in the biggest study of its kind.
The respected Ho Chi Minh Pasteur Institute in Vietnam said it had decoded 24 samples of the H5N1 virus taken from poultry and humans.
The results showed a significant variation of antigen - any foreign substance that stimulates the body’s immune system to produce antibodies.
The H5N1 type that infected people and waterfowl in early 2005 has several mutations focusing in the important functional parts of the surface proteins.
The study also found a mutation of the PB2 gene in a virus sample from a patient who died in the southern Dong Thap province earlier this year.
Small changes in this gene can make it much easier for the virus to breed in humans making it more deadly.
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