In 1918, a deadly influenza pandemic swept through the world, killing millions of people. Although the exact number of deaths is not known, estimates now put the toll somewhere between 21 million and as high as 50 million people, the highest death toll of any disease outbreak in history. This influenza was unusual in that the majority of its victims were young, and largely because of the epidemic’s violent “second wave”, which was spread in part from the close contact among the troops in World War I, it produced a worldwide pandemic.