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.

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Given the virus’ propensity for mutation and its constant presence in nature (it occurs naturally in wild water birds), experts agree that it is only a matter of time until a strain emerges that is just as contagious and deadly as the Spanish flu – and possibly even worse.

“Influenza pandemics are like earthquakes, hurricanes and tsunamis: they occur, and some are much worse than others,” says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “The idea that we would not have another 1918-like event is foolish.”

But when that will happen, he continues, is impossible to predict: “For all we know, it could be starting as we speak.” It’s also impossible to predict exactly how things will play out when a Spanish flu-like strain does reemerge – but we can make some educated guesses.