It was in the fall of 1918 that a group of sailors from the United States began presenting never-before seen symptoms that baffled clinicians. The presenting symptoms were bleeding in the nose and ears, pounding headaches, painful body aches, deep coughs (sometimes deep enough to tear abdominal musculature), and, finally, skin turning blue.

The medical professionals called in Paul Lewis, a lieutenant commander and a medical doctor who was more familiar with death in all its varieties than just about anyone alive at the time. He was also brilliant. More than a few colleagues—accomplished scientists in their own rights—called Lewis the most brilliant man they’d ever met.

Lewis was highly accomplished and still young. When polio had ravaged New York, he was part of the group that proved a virus was the culprit. And then he developed a vaccine that proved 100 percent effective in animal trials. He’d also founded a research institute in affiliation with University of Pennsylvania. Though Lewis was an accomplished man of science and familiar with death and all its friends, he was still baffled by the bodies of dying, blue-skinned sailors. He’d seen something similar among British soldiers weeks earlier, something influenza-like, but he wasn’t sure.

Whatever it was that the sailors who came through Boston to Philadelphia had brought with them that fall, it spread. Despite the best attempts of medical personnel to contain the unknown disease, it spread from the 19 soldiers to 87, and then to 600 within just a few days. Hospitals ran out of beds quickly and had to involve other medical facilities to care for the sick sailors and the civilians with whom they’d come in contact. Simultaneously, the same symptoms began showing up all over the world. This wasn’t a passing rash of influenza going around as doctors in the United States and Europe had thought. It was actually the second wave of a mild influenza that had appeared months earlier in America’s heartland. It was not nearly as devastating then, in symptoms or spread. The second, far more pernicious infection was spreading like wildfire in the fall of 1918, affecting not just sailors in New England, but also soldiers in the British Raj in India, and everywhere in between.

And so, as the Great War continued to rage, another war had begun. It wasn’t just a fight of nation against nation, but also of nation against some unknown disease. It began in a small town in the United States in the spring of 1918, but had laid dormant. But between the fall of 1918 and 1920, millions died. Of those who contracted influenza and succumbed, their deaths were swift and painful.

Earlier estimates put the death toll at 21 million, but this is now considered a low ball. More widely accepted estimates from epidemiologists are between 50 and 100 million. The majority of these deaths took place within a half-year window, during the fall and winter of 1918. The disease killed more people in a year than ever died in the medieval Black Plague or from AIDS.

What made this pandemic even more tragic was that it was the young (people in their 20s and 30s) who were especially vulnerable to the pathogen. If the higher estimates are accurate, that means the 1918 influenza took out about 10 percent of young adults on the planet.

The 1918 outbreak marked a milestone in human history. It was the first time that modern medicine and nature had challenged each other in such a robust way. The virus that led to the infamous Bubonic Plague 700 years earlier was a far milder strain, but it still decimated Europe because science and infrastructure couldn’t put up any real fight. This time, it was different, and it was the individuals who retained poise and calm in the midst of dire circumstances who stopped the bleeding and kept the catastrophe from being any more grim than it already was.