Health experts have discovered troubling similarities between one of history's deadliest pandemics and the current H5N1 bird flu strain spreading in Asia.

A team of scientists completed the astonishing feat of rebuilding the virus that caused the Spanish Flu, which swept the globe in 1918 and killed up to 50 million people (including some 50,000 Canadians).

"We felt we had to recreate the virus and run these experiments to understand the biological properties that made the 1918 virus so exceptionally deadly," said Terrence Tumpey of the U.S. Centers for Disease Control and Prevention, who helped write the reports published jointly this week in the journals Nature and Science.

The scientists began the task of piecing together the complete protein-coding sequence of the virus in 1995.

They found the 87-year-old virus in bits of lung tissue preserved from victims of the epidemic. They were able to rebuild the strain using all eight of the Spanish flu's viral genes and mixing it with noncoding DNA from another flu virus.

The strain replica is now being stored in a biosafety Level 3 lab at CDC headquarters in Atlanta, Georgia.

As Dr. Jeffery Taubenberger reports in this week's Nature, the newly sequenced genes -- encoding proteins called polymerases, crucial for viral replication in human cells -- bear striking similarities to those of flu viruses found only in birds.

The 1918 flu strain was also originally an avian flu. But it somehow didn't mix with any human flu strains before it jumped directly to people to become deadly and highly contagious.

This is in contrast to the flu viruses that caused pandemics in 1957 and 1968, both of which probably combined with human-adapted strains before becoming killers, said the authors.

"We now think that the best interpretation of the data available to us is that the 1918 virus was an entirely avian-like virus that adapted to humans," Taubenberger told reporters in a telephone briefing.

"It suggests that pandemics can form in more than one way."

The current H5N1 strain of bird flu has swept through poultry populations in large swaths of Asia since 2003. It has jumped to humans and killed at least 65 people -- more than 40 of them in Vietnam -- and resulted in the deaths of tens of millions of birds.

At this point, H5N1 doesn't infect humans easily but because it is an avian strain, humans have no immunity to it and it can kill quickly.

Taubenberger found that the 1918 flu had undergone several mutations in each of its genes. The H5N1 flu is beginning to show some of the same changes too, he said, but appears to be early on in the process.

But since the scientists that revived the Spanish flu are now beginning to understand what those mutations are, they can perhaps start work on antiviral drugs and vaccines that can fight them and maybe even head off the next pandemic.

"We want to derive lessons from what we study about the 1918 virus to help us understand how influenza pandemics might form for the future," said Dr. Taubenberger. "And what we might be able to do to prevent them."

Kirsty Duncan, a medical geographer who has written a book called Hunting the 1918 Flu told Canada AM that any research that helps us to understand the flu is always welcome.

"I think it would surprise most people to learn that influenza remains one of the greatest unconquered threats to human health today," she says.

"We spend $10 billion each year fighting it in the Northern Hemisphere. So this is important, exciting research that really is going to better our health in the long term."