By Michelle Roberts
BBC News Health reporter, in Washington DC
The bird flu virus could mutate to pass from human to human and trigger a pandemic, latest evidence suggests, according to scientists.
There are 15 different strains of the virus
Outbreaks so far have been through the flu spreading from animals to humans.
But Nancy Cox, of the US Center for Disease Control and Prevention, says a number of subtypes have proven their ability to jump the species barrier.
The H5N1 strain, which has killed 42 people in Asia since 1997, was one of many possible candidates, she said.
Strains had emerged in the last year that were more lethal to animals than the 1997 strain, she said.
The recent spurt of human infections increases the likelihood that a mutant strain would arise that could spread between humans, she added.
"It's impossible to predict what the consequences would be. We might have a relatively mild pandemic like we did in 1968," Dr Cox told the annual meeting of the American Association for the Advancement of Science (AAAS).
"Alternatively, we could have a relatively severe pandemic as occurred in 1918 or perhaps even worse."
The virus could mutate by shuffling genetic material with the human flu virus, Dr Cox added.
This would make it better at specifically targeting human airways for attack.
During the last century there have been three serious flu outbreaks.
The first in 1918, dubbed the Spanish flu, killed up to 50 million across the world.
Asian flu hit in 1957, followed by Hong Kong flu in 1968, which claimed one million victims each.
Although H5N1 has only killed 42 people so far in comparison, its death to infection rate is 76%.
"It is very frightening to see such a high case fatality rate," said Dr Cox, but she said it might be that less serious or ambiguous cases had not been picked up, which would mean the real rate could be lower than this.
A study published in this week's New England Journal of Medicine supports this notion.
The Vietnamese authors report a case of a four-year-old boy who had diarrhoea and seizures rather than respiratory symptoms before going into a coma and dying from the H5N1 virus.
Dr Cox said it was important to keep a handle on what the virus was doing and for countries to prepare for a pandemic by stockpiling antiviral drugs.