One hundred years ago nearly to the day, flu season seemed to be wrapping up without further incident. Most of those who had fallen ill in the spring had made a speedy recovery, and death rates were no higher than usual. Global headlines were dominated by news about the Great War, not the flu.

Come autumn, however, everything changed. The formerly unexceptional virus reappeared as an exceedingly virulent strain, tearing through populations in North America and Europe and often killing its victims in a matter of hours or days. Within four months, the Spanish Flu, as it came to be known, had spread around the world, making its way to even the most isolated communities. By the time the pandemic flamed out the following spring, an estimated 50 to 100 million people – as much as 5% of the world’s population – were dead.

A century later, the 1918 pandemic seems as remote a horror story as that of smallpox, the bubonic plague and other deadly diseases that we have wholly or largely eradicated. Yet influenza never left – it continues to claim some 250,000 to 500,000 lives annually. Each year delivers a slightly different strain of the seasonal flu, while pandemics may arise by an assortment of influenza viruses in animal hosts. In addition to 1918, the last century has seen pandemics in 1957, 1968, 1977 and 2009.

Click here to read more about the possible effects a modern pandemic.