Bird flu could be "worse than Sars", if it mutates so it can spread between humans, experts have warned.
The disease is picked up through close contact with poultry
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."
Up to 12 people are thought to have been infected with avian flu in the Vietnam outbreak, including the three who have died.
In a Hong Kong outbreak in 1997, the disease killed six out of 18 people who were infected.
Sars has killed around 800 people worldwide and infected at least 8,400 since it first emerged in November 2002.
Dr. Shigeru Omi, WHO regional director for the Western Pacific, added: "The evidence to date is that there is no sign of human-to-human transmission."
"However, should this occur, we would have a serious situation. "
It is thought that around a million birds have died from the virus, with thousands of others being culled to try and prevent the spread of the disease.
Vietnam last week confirmed it had seen an outbreak of the illness among chickens.
South Korea also confirmed its first case of the virus in birds for a week. It had already culled around two million chickens and ducks following am outbreak in December.
The disease has also been seen in chickens in Japan.
Various countries have announced bans on the import of live birds from affected areas.