"A bout of the flu can be mild. In young, healthy adults, many infections pass unnoticed. But sometimes the influenza virus evolves into a strain that decimates its victims. The worst known strain swept the world in the Fall of 1918, infecting 500-1000 million and killing 40-100 million, about 2-5% of people."
The World Health Organisation is forecasting another major flu epidemic in the near future.