The Spanish flu virus that killed up to 50 million people in 1918-19 was probably a strain that originated in birds, research has shown.
Flu viruses are constantly evolving
US scientists have found the 1918 virus shares genetic mutations with the bird flu virus now circulating in Asia.
Writing in Nature, they say their work underlines the threat the current strain poses to humans worldwide.
A second paper in Science reveals another US team has successfully recreated the 1918 virus in mice.
The virus is contained at the US Centers for Disease Control and Prevention under stringent safety conditions.
It is hoped to carry out experiments to further understand the biological properties that made the virus so virulent.
The virus was recreated from data produced by painstaking research by a team from the US Armed Forces Institute of Pathology.
Lung tissue samples
Working on virus samples from the remains of victims of the 1918 pandemic, the researchers were able to piece together the entire genetic sequence of the virus.
They found the virus contained elements that were new to humans of the time - making it highly virulent.
And analysis of the final three pieces of the virus' genetic code has revealed mutations that have striking similarities to those found in flu viruses found only in birds, such as the H5N1 strain currently found in south east Asia.
This strain has so far killed at least 65 people.
Many experts believe it is only a matter of time before H5N1, or a similar strain, causes many deaths in humans - possibly after combining with a human flu strain.
Crucially, the mutations identified by the US researchers were found in genes which control the virus' ability to replicate in host cells.
The researchers say these mutations may have helped the 1918 virus replicate more efficiently.
At this stage, they say the H5N1 strain shares only some, and not all, of these mutations.
But these mutations may be enough to increase the virus' virulence - and give it the potential to cause serious human infection without first combining with a known human flu strain.
The researchers believe the two other major flu pandemics of the 20th century - in 1957 and 1968 - were caused by human flu viruses which acquired two or three key genes from bird flu virus strains.
But they believe the 1918 strain was probably entirely a bird flu virus that adapted to function in humans.
Julie Gerberding, director of the US Centers for Disease Control, said: "By unmasking the 1918 virus we are revealing some of the secrets that will help us predict and prepare for the next pandemic."
And Dr Jeffery Taubenberger, lead researcher of the Nature study, said: "Determining whether pandemic influenza virus strains can emerge via different pathways will affect the scope and focus of surveillance and prevention efforts."
Professor John Oxford, an expert in virology at Queen Mary College, London, said the suggestion that the virus had the potential to jump between humans without first combining with a human virus made it even more of a threat.
"This study gives us an extra warning that H5N1 needs to be taken even more seriously than it has been up to now," he said.
Dr Terrence Tumpey, of the US CDC, defended the decision to recreate the 1918 flu virus.
He said: "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.
"We wanted to identify the specific genes responsible for its virulence, with the hope of designing antivirals or other interventions that would work against virulent pandemic or epidemic influenza viruses."