Deadly mutations:The Spanish influenza virus that killed 50 million people in 1918-19 probably originated in birds, according to researchers. U.S. scientists have found the 1918 virus shares genetic mutations with the bird flu virus now circulating in Asia.

Writing in the journal Nature the scientists say their work underscores the threat the current strain poses to humans worldwide. A second paper in Science reveals another U.S. team has successfully re-created the 1918 virus in mice. The virus is contained at the U.S. Centers for Disease Control and Prevention under stringent safety conditions for experiments to understand what made the virus so virulent.

The virus was re-created from virus samples from the remains of victims of the 1918 pandemic, enabling researchers 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. Analysis of the virus’ genetic code revealed mutations with similarities to those in flu viruses found only in birds, such as the H5N1 strain.

Experts believe it is only a matter of time before H5N1 or a similar virus causes many deaths in humans — possibly after combining with a human flu strain. The mutations identified by the U.S. researchers were found in genes that control the virus’ ability to replicate in host cells. Researchers say these mutations may have helped the 1918 virus to replicate more efficiently.

At this stage, the H5N1 strain shares only some, not all, of those mutations. But the mutations may be enough to increase the virulence of the virus, 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 that 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.