Deadly genes from the human influenza virus which caused the 1918 pandemic have been resurrected by scientists in an attempt to understand what made the strain so virulent.

The "Spanish flu" - which may have killed as many as 50 million people in the great pandemic - owed its deadliness largely to one of its surface proteins, say the researchers.

They have tested reconstructed viruses equipped with the protein and warn that it might one day return. If it does, anyone born after 1918 will have virtually no immunity.

Yoshihiro Kawaoka of the University of Wisconsin in Madison, US, and colleagues have reconstructed five of the eight genes of the 1918 virus, from fragments in victims buried in permafrost, and from clinical samples.

They reconstructed the two surface proteins - haemagglutinin (H) and neuraminidase (N) - from the 1918 flu strain, and substituted them for the H and N of current flu strains from mice and humans.

The 1918 H even made a virus that normally kills mice even more lethal. In every case, the 1918 hybrid replicated more, invaded the deeper tissues of the mouse lungs, and caused nastier lesions.