Between 1918 and 1919, an outbreak of influenza spread rapidly across the world, and killed more than 50 million—and possibly as many as 100 million—people within 15 months. The speed of the pandemic was shocking; the numbers of dead bodies overwhelmed hospitals and cemeteries. Quarantine centers, emergency hospitals, public use of gauze masks, and awareness campaigns were all undertaken swiftly to halt the spread. But as World War I was coming to a close, millions of soldiers were still traveling across the globe, aiding the spread of the disease. While its exact origins are still debated, it’s understood that the “Spanish Flu” did not come from Spain. The name seems to have arisen as reporting about influenza cases was censored in war-affected countries, but Spain was neutral, so frequent stories appeared about the deadly flu in Spain. Gathered here are images from the battle against one of the deadliest events in human history, when the flu killed up to 6 percent of the Earth’s population in just over a year.