 It's the early part of the 20th century. You step out of your cottage in the small farming town of Sedan in the north of France. You look around and smell the aroma of freshly made baguettes floating along in the wind. Through the air, you hear a slight whistling sound. Before you know it, a mortar lands at your feet. Oh yeah. The Germans invaded France in 1914 and Blitzkrieg toured Paris until they were stopped at the Miracle of the Marne in September. To prepare for a second assault, the French dig trenches along the battlefront throughout Belgium and northern France and the Germans quickly follow suit. Now, what's left of your small village is tucked between the second and fourth largest armies in the world. To put it simply, you're screwed. But life didn't fare much better elsewhere in the world. With the Wright Brothers' first powered flight in 1903, the invention of the Model T in 1908 and the explosion on World Trade at the end of the 1800s, you'd be forgiven for thinking that the turn of the century would be a time marked by an increased connection between the world's people, lending itself to a new era of cultural exchange and prosperity. But the turn of the century and the ever-increasing connectedness of mankind brought with it new conflicts that were killing millions of people across the globe. The Russo-Japanese War from 1904 to 1905 preceded the even deadlier Russian Revolution in 1905. The Turkish-Italian and Balkan Wars tore southern Europe apart from 1911 to 1913, and a list of failed coups and uprisings dotted the globe. It seemed an ever more connected world was failing to bring peace, but rather facilitated conflict. With a complex network of interconnected alliances throughout Europe, the stage was set for a great conflict. And with Austria-Hungary's declaration of war on Serbia in July of 1914, the Great War had just begun. Western powers were eager to jump into the ring with their newly developed weapons of war like tanks, nerve agents, and rudimentary bombers. But while mankind's capacity to destroy surged ahead at breakneck speeds, our capacity to rebuild was decidedly laggard. Modern medicine wasn't even in its infancy. Antibiotics wouldn't first be discovered for another decade. Night even 50 years prior had doctors first recognized the need to sterilize their instruments or even to wash their hands. And while doctors on the front lines of World War I knew the importance of keeping wounds clean, in the field this was limited to filling wounds with salt or iodide until patients could be transferred out of the battle zone. In the trenches, factors like proximity, weather, dirt, and trench fever created the perfect recipe for six soldiers. Meanwhile, increasing populations across the globe, especially in poorer, third world countries with little to no access to the medical technologies of the day, put great strain on what little health resources there were. All these factors were the kindling for what was to come. Some say it started in China, while others think it originated in the US. Regardless of where you think it came from, it seemed to spread quickly and unnoticed, reaching small islands dotted across the Pacific, researchers in the Arctic, and even infected Greenland. And for anyone that's played play gink, you know how hard infecting Greenland can be. Many developed nations trying to keep citizens' morale high during the Great War censored the news from reporting on the sick that seemed to be piling up, afraid of the panic that could ensue if the public were aware of the fast moving plague sweeping through their country. This made it difficult for anyone to connect the dots and realize this wasn't a simple illness but a growing epidemic and no one had yet realized its global reach. But Spain, not participating in the war, was less concerned with keeping morale high and so papers were free to report the epidemic's effects on the country. Unfortunately, this gave the impression that Spain was especially hard hit and the country would wind up bearing the disease's name. The flu would come to infect 500 million people around the world, nearly one in three people living in 1918 and killed an estimated 15 million people, making it the second deadliest plague in history. It killed 25 million people within the first 25 weeks alone. That's the equivalent of killing the entire city of Dallas, Texas every week, although maybe that would finally get rid of the cowboys. A normal flu is fatal in less than 0.1% of cases, but for the Spanish flu it was 10%. But what made the Spanish flu so dangerous? To answer that, you need to know a bit about how the flu virus works. The flu's proper name is influenza and there are four subtypes, from influenza A to influenza D and type A is the main type that causes pandemics. The viruses are categorized by two surface proteins, hemigladen, which comes in 18 varieties, and neuraminidase, which comes in 11. This is what gives flu's their coded names, like the Spanish flu's H1N1. Your immune system builds up antibodies that recognize all of these proteins and can protect you from multiple infections, but the influenza virus can mutate so quickly that your body can't always keep up. Most mutations are very small, so small that your body will probably be able to defend itself against a mutated strand. But the 1918 flu virus underwent a much bigger change that left many people vulnerable. Big mutations like these can result in viruses with very different or even entirely new combinations of surface proteins, and they're so different that most people have little or no natural immunity. Big changes like these are what cause flu pandemics. Even though the virus changes all the time, today scientists think that the first flu you encounter might have a lifelong effect on the way your body fights new flu strands, sort of like your body setting up the first fluid encounters to its default state. This means that if you encounter the same strand later in life, you'll be especially effective at fighting it off, even if it's changed a little, but you won't do as well against different flu strands with different surface markers. This is exactly what caused the Spanish flu to be so deadly 100 years ago. In the end of the 1800s, the so-called Russian flu was the dominant strain. And that was probably the H3N8 variant. Then around 1900, the H1 type became dominant, and sometime right before 1918, another shift created the H1N1 strain. This meant that people born after the Russian flu had nearly no natural immunity to an H1N1 type flu, like the one that was spreading across the globe. And oddly enough, those people's young, healthy immune systems actually worked against them and caused a lot of them to die quickly after showing symptoms. These rapid deaths were caused by cytokine storms, which is an immune system response so intense that it can damage a person's tissues. And the stronger a person's immune system is, like the ones found in young people, the more damage a cytokine storm can do. So in a weird twist of fate, the healthiest people who are normally the most immune to the flu were actually the ones who suffered the most from it. So even with student debt and rising housing prices today, 1918 was not a great time to be a young person. After the outbreak of the Spanish flu, a huge portion of the global population had gained an immunity, and the virus probably mutated to a less deadly form, which helped save off further chaos. The scary thing is, even today, scientists aren't exactly sure what magical combination of mutations makes a flu more or less deadly. Either way, these days, we don't need to worry about the Spanish flu specifically. Many of today's H1 viruses are actually descended from that 1918 virus, so most people have at least some H1 immunity. But if there's ever another big mutation, that could be a problem. To protect us from that possibility and all other flu seasons, scientists are working on a universal flu vaccine. There are several in development right now that target parts of the virus that are the same across all variants, rather than just focusing on the surface proteins of individual strains. And if any of them work, it could someday mean that one vaccine could protect you from every flu strain across every season. So even though it's possible, hopefully we'll never experience another pandemic like the Spanish flu ever again. Thanks for watching this episode of Everything Science. This idea was actually recommended by a subscriber. So if you have any ideas for videos you want us to cover, make sure to leave those in the comment section down below. And remember, there is always more to learn.