Bird flu could be "worse than Sars", if it mutates so it can spread between humans, experts have warned.
Three deaths in Vietnam have been linked to the disease, which is affecting poultry across Asia.
Experts say that all cases seen so far in humans have occurred after people had been in close contact with sick birds.
But the World Health Organisation has warned that avian flu has a higher mortality rate than Sars.
Peter Cordingley, spokesman for the World Health Organization regional office in Manilla told the AFP news agency: "If the H5N1 virus attaches itself to the common human flu virus and if it is then effectively transmitted, it has the potential to cause widespread damage. The common human flu virus is far more infectious than the Sars virus and can spread by aerosol and not just through droplets as in the case of Sars virus."
Mr Cordingley added: "The mortality rate is far higher than that of the Sars virus."
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