Scientists are keeping a close eye on a new influenza virus found in Hong Kong that is similar to strains that killed more than 22 million people this century.
So far, the death of a 3-year-old boy in Hong Kong in May is an isolated case. But health organizations around the world have been alerted because the potential of the new virus for causing a pandemic, or global epidemic, in humans in unknown.
"I do not think it is a serious threat, not at this moment," Albert Osterhaus of the National Influenza Center of the Netherlands said in an interview.
"But these viruses have, in principle, potential to be pandemic viruses."
More than 20 million people died in the world's worst global flu epidemic -- the Spanish flu of 1918. Nearly 40 years later, one million succumbed to the Asian flu, and in 1967 the Hong Kong flu claimed 700,000 lives.
Scientists do not know what causes a flu pandemic. They believe the new strain originates in birds or pigs and then crosses over to humans who have no immunity against it.
Advances in technology have allowed them to identify new lethal viruses in humans and to launch global warning systems and immunization programs.
Many virologists believe another pandemic is already overdue.
Osterhaus and his colleagues examined the virus that was isolated from the respiratory tract of the boy who died in May. They confirmed that it was a new virus that had not been seen in humans before.
In a letter published today in the scientific journal Nature, he explained that the viruses responsible for earlier pandemics involved strains from birds that carried a combination of two viral envelope glycoproteins, haemagglutinin (H) and neuramidase (N).
The H1N1, H2N2 and H3N2 combinations caused the previous three major epidemics.
The new virus, which virologists identified as H5N1, has a similar combination, and doctors believe the child may have caught it while visiting a chicken farm a week before he became ill.
Experts from the World Health Organization are investigating a similar virus that has killed chickens in Hong Kong to determine if the boy's case was a direct spillover and if there are other people infected with it.