Not only did the Spanish flu's H1N1 virus cause tens of millions of deaths in 1918, it was also transmitted from humans to pigs during the pandemic and continues to evolve today, the authors of the study said.

"The 1918-1919 influenza pandemic was a defining event in the history of public health," said Anthony Fauci, the director of the National Institutes of Allergy and Infectious Diseases and a co-author of the study.

"The legacy of that pandemic lives on in many ways, including the fact that the descendents of the 1918 virus have continued to circulate for nine decades," he said.

The authors of the study said that since it first appeared in 1918, the virus "has drawn on a bag of evolutionary tricks to survive in one form or another -- and to spawn a host of novel progeny viruses with novel gene constellations, through the periodic importation or exportation of viral genes."

Jeffrey Taubenberger, the senior investigator at NIAID's Laboratory of Infectious Diseases, said all human-adapted influenza A viruses "are descendents, direct or indirect, of that founding virus."

"Thus we can be said to be living in a pandemic era that began in 1918," he added.

All flu viruses have eight genes, including two that are coded to produce the proteins hemagglutinin (H) and neuraminidase (N) that allow the virus to enter a host cell and spread from cell to cell, the report said.

There are 16 H sub-types and nine N sub-types, making 144 possible HN combinations.

But only three -- H1N1, H2N2, and H3N2 -- observed to date are fully adapted for infecting humans.

Other combinations such as the H5N1 bird flu virus have only occasionally infected small numbers of humans.

"The eight influenza genes can be thought of as players on a team: Certain combinations of players may arise through chance and endow the virus with new abilities, such as the ability to infect a new type of host," said David Morens, an author of the study.

That is likely what caused the 1918 pandemic, he added.

Through a still-unknown mechanism, the virus gained an ability to infect people and to spread rapidly, the study said.

Although the viral dynasty of the 1918 flu is far from becoming extinct, the virologists note that there are reasons for optimism.

The pandemics that have occurred since 1918 have gradually diminished in severity.

The Asian flu of 1957 and the Hong Kong flue of 1968 caused four and two million deaths respectively while so far only 311 people have died of the current swine flu pandemic, according to the latest WHO figures released Monday.

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