Scientists who recreated"Spanish flu" - the 1918 virus which killed up to 50m people - have witnessed its remarkable killing power first hand. The lungs of infected monkeys were destroyed in just days as their immune systems went into overdrive after a Canadian laboratory rebuilt the virus.

The reason for the lethal nature of the 1918 flu was never fully understood.

But the experts behind this test say they have found a human gene which may help explain its unusual virulence.