In 1918, at the end of the World War I, a flu pandemic ripped through the world with such speed and virulence that by the end of the following year an estimated 40 million people, four times more than those claimed by the war. Where did this particular flu strain come from and what made it so deadly? "Killer Flu" will show how 85 years later, virologists and epidemiologists are still hunting down the answers to those two critical questions.