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American policeman wearing a mask to protect himself from Spanish flu, ca. 1918.
Despite gaps in the historical record, it is believed that flu pandemics have occurred throughout human history, when especially nasty strains of influenza virus spread far and wide, sometimes affecting humanity on a global scale.
In the past 100 years, four flu pandemics have spread across the earth. It is estimated that the worst of these pandemics killed 50 million people, and the other outbreaks, while less serious, have still left millions dead.
While flu season appears to be winding down this year, it remains a serious disease that has wreaked havoc on humanity in the past century. Here is a look at each of those pandemics.
