Flu's deadly secret revealed

A US-Japanese research team recently announced it had isolated three genes that explain why the 1918 Spanish flu, believed to be the deadliest infectious disease in history, was so lethal.

The pandemic killed between 20 and 50 million people - more than in all of World War I, which ended in November 1918 - and spread around the world.

The genes allowed the virus to reproduce in lung tissue, according to the new research.