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We've got a problem. A big problem that's been bothering humans for a really long time. You might even say that this problem has been… plaguing us. Yeah, it's the plague. More specifically the bubonic plague, of "Black Death" fame, during the Middle Ages. But that was neither the first nor the last time it afflicted us. In fact, people are still getting plague to this very day. So let's explore the ancient history of humanity's most infamous disease. [♪INTRO] Scientists discovered the cause of bubonic plague back in 1894.
It wasn't the wrath of god; it wasn't an astrological misalignment of the planets. It's a type of bacteria called Yersinia pestis, or Y. pestis if you're in a hurry. You'll notice that 1894 is several hundred years after Europe's medieval era. So before then, people also used the term "plague" pretty willy-nilly, to describe both globe-spanning pandemics and personal annoyances. But these days, "plague" means Y. pestis, which comes with fever, chills, pain in your limbs, and… when the bacteria infect your lymph nodes… the infamous lumps called buboes.
You can also say buboes, and therefore the bubonic plague. Which I think sounds a little more appropriate, because ew. And that's right. Y. pestis actually causes different plagues, depending on where it infects you. In addition to bubonic plague, there's also septicemic plague, which infects your blood and can lead to blackened tissues from gangrene, and pneumonic plague which gets into your lungs. For the record, the bubonic variety is actually the most survivable of the three. Despite knowing quite a lot about plague, for quite a long time, scientists are still finding out new information about this very old disease. In 2021, one team found evidence for the oldest known case of plague:
a man who lived in present-day Latvia more than 5000 years ago. The researchers analyzed the ancient Y. pestis DNA from the man's remains, and they concluded the strain probably wasn't super contagious. Nevertheless, the man probably got infected by a rodent bite, had septicemic plague, and died a few days later. Then, in 2023, another research team tracked the plague's progress to Britain, 4000 years ago. They found Y. pestis DNA in the teeth of people buried at two different sites on the island, about a four and a half hour drive apart.
This specific strain of plague is called the Late Neolithic Bronze Age lineage, or LNBA for short. Between roughly 2900 to 500 BCE, it appeared all over Eurasia, from Mongolia to Western Europe. But it's a bit unclear how it managed to do that. This seems to be a few centuries before Y. pestis developed an adaptation that vastly improved its ability to hitch a ride and spread via flea bites. That adaptation was a brand new gene, called ymt, which one lucky Y. pestis cell acquired from an interaction with some other microbe's DNA, and then passed it on to all its descendants. ymt helps the bacteria survive in a flea's digestive tract. While Y. pestis was still infectious without it,
it wasn't infecting people directly through flea bites as much. A different study, published in 2025, may help explain things. Because researchers found some LNBA-era plague DNA in the bones of a sheep from the modern-day Russian region of the Eurasian steppes. This was a domesticated sheep, and its variety of plague had a close genetic match to samples from infected humans who lived in the same region around the same time. It's likely this sheep came into contact with an infected wild animal. The plague then passed from the sheep to humans when the sheep was being handled and consumed. So Y. pestis may have originally infected most humans via
their livestock. But eventually, fleas got their time to shine. Researchers haven't found the genetic mutation required for flea transmission in any Y. pestis samples more than 3,700 years old, but it's extremely common in those younger than 3,000 years. But be it a sheep, or be it a flea, a spillover from animals to humans is a good way to start a pandemic. And in 541 CE, we got the start of our first proper plague pandemic. The Plague of Justinian, named after the Byzantine emperor at the time, is often considered to be both the first plague pandemic and the first recorded pandemic, ever.
It probably started around Egypt and the Mediterranean, and it killed anywhere between 25 and 100 million people. Contemporary accounts of the symptoms led scientists to speculate this pandemic was caused by Yersinia pestis. They also uncovered contemporary samples of Y. pestis, but some researchers felt these weren't close enough to the outbreak's supposed epicenter to be definitive proof. It wasn't until 2025, and the bacterial DNA hiding in eight teeth unearthed from a mass grave in Jordan, that they decided, "Yeah; it was definitely plague, guys."
Now, the Plague of Justinian was not a one-and-done incident. The initial round of outbreaks lasted until about 549 CE, but the particular strain of Y. pestis behind it stuck around for two more centuries. It triggered another 18 or so waves of plague outbreaks. Then, it disappeared. This seems to be a common pattern for plague. Any given strain tends to die out after just a few centuries, most likely due to anyone with a natural genetic resistance to the disease surviving and passing that onto their offspring.
Over time, this would increase the population's overall resistance, reducing disease spread. But as history knows, Yersinia pestis wasn't even close to being done with us. You know what's also not done with us? The economy. So here's an ad. Gracias, Babbel for supporting this SciShow video! Yesterday, I was on a train. I ended up sitting next to a Mother and son who spoke Spanish with each other, navigating a train that made all announcements in English. And they spent the whole ride playing on an app that taught them yet another language!
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I could go on Babbel to learn what that mother meant when she said "¿Cuánto falta para llegar?" Your language learning can help you have practical real world conversations about travel, business, relationships, and even where to find the restroom. When you click the link in this video description or scan the QR code on screen, you'll get 55% off your Babbel subscription and learn why Babbel is one of the top language learning apps in the World! In 1346, the Black Death kicked off humanity's second plague pandemic. Scientists have tried to track down the origins of the Black Death for centuries. The stories used to go that it started in China, and quickly spread down the Silk Road to Europe.
It turns out those stories were just that: stories, originating from a 14th century poet who wrote about a fictional trickster spreading the plague from China to the Mediterranean. Even though this particular tale was never meant to be taken as fact, its author did later write historical pieces. People apparently mistook his trickster tale for history as well. So if you've ever felt bad because a movie taught you something very wrong about history, take comfort that one of your ancestors would experience something similar.
Fortunately, we now have modern tools to analyze ancient Y. pestis DNA. And by analyzing how mutations accumulate over time and space- and the power of statistics- researchers can identify where certain genomes are most likely from. In a study in 2022, researchers collected ancient DNA from 14th century plague victims. They sequenced the plague genomes that they found in the bodies, and compared those to other genomes of known geographical location. Turns out, this strain of plague probably got its start in Central Asia. Not China. Even though the Black Death only lasted a handful of years, it managed to wipe out at least one third of Europe's population.
So many people died that the remaining population passed along a very particular set of genes that protected their descendants. Which is great, although not always. Some versions of these plague-protecting genes are also associated with an increased prevalence of autoimmune disorders. So if you've got Crohn's Disease and a European ancestor, congrats on probably being a direct descendant of a plague survivor! Now, I did say the Black Death kicked off the second plague pandemic. In fact, that pandemic didn't end until the 1800s. And genetic evidence reveals Y. pestis probably re-entered Europe multiple times over the centuries.
In which case, there was probably a reservoir of this specific strain outside Europe, likely involving some kind of rodent. A reservoir is a niche in the environment where a disease can hang out, multiply, and continue existing in the wild. They're super important for understanding the existence and spread of plague. because it's not just the fleas that we have to worry about. It's also the animals that the fleas feast on. With the movement of both people and animals, the Black Death strain kept circulating throughout Western Europe.
Along the way, natural selection did its thing, and one Y. Pestis strain split into two major lineages. One of them is the evolutionary ancestor to all modern strains of plague… which we'll get to in a moment. The other lineage died out by the 1800s. The exact reasons for this extinction aren't totally clear. Some scientists think this branch of the Y. pestis family tree evolved to hang out in an animal reservoir that no longer exists. Another hypothesis has to do with the genetics of the plague strains themselves. In both of the pandemics we've talked about so far, the Y. pestis strains evolved to be less deadly after about 100 years.
According to a paper published in 2025, this decrease in mortality is associated with fewer copies of a gene called pla. The pla gene helps Y. pestis fly under your immune system's radar until it reaches your lymph nodes. So if you're Y. pestis, it sounds pretty nifty to have as many copies of pla as possible in your DNA, right? Well, when the plague strain has more copies of pla, it's more deadly more quickly, so the host doesn't have as much time to spread the disease to others.
Meanwhile, when there are fewer copies of pla, the host lives longer and the disease is less severe, so it can pass on to more hosts. In other words, there's an evolutionary advantage for the bacteria to stop having so much pla. But not everyone agrees with this pla hypothesis, so the jury's still out. FYI, the fact checker for this script noted that we missed out on saying this hypothesis was pla-sible. Thanks, Angela. As for the Black Death lineage that kept on cooking, in the late 19th century, a third plague pandemic broke out in Yunnan, China.
It spread all across Asia, then globally until the 1950s, causing 12 million deaths in a century. Thankfully, we are not currently in the middle of a plague pandemic… as of the day we're filming, at least. But that doesn't mean humanity has had its last one. The plague is definitely still around in the 21st century. We've just traded our plague doctor masks for N95s. Today, Y. pestis is most commonly found in Madagascar, the Democratic Republic of the Congo, and Peru. Thankfully, even though the plague comes and goes in those areas, the number of infections tend to stay pretty low: hovering under 50 cases, each.
The United States sees about seven plague diagnoses per year, and extremely low death rates. But in July 2025, a person from Arizona died of pneumonic plague. While the official report didn't share how this person got the plague in the first place, animal reservoirs for Y. pestis in the US tend to be rodents, especially prairie dogs, squirrels, and chipmunks. I don't care if they're cute, DO NOT TOUCH THE WILD ANIMALS! So, what tools do we have to fight and prevent plague? If you've ever seen Monty Python and the Holy Grail, the monks going around bashing themselves in the head
are probably loosely based off of a real-life anti-plague belief during the Black Death. People thought that the plague was the wrath of God, and hitting themselves was a way to punish themselves to earn forgiveness. Fortunately, we have more effective ways of dealing with plague. Masks do prevent transmission, at least the human-to-human kind, and we have plenty of antibiotics that are effective if they're given early enough. Unfortunately, some strains of Y. pestis have already developed resistance to antibiotics, but that doesn't seem to be an immediate concern to health professionals.
They're focusing on other bacteria that are already showing anti bionic resistance at the moment. Meanwhile, we don't have many vaccine options available, but scientists are working on it. In this case, it tends to be a challenge of making a vaccine that works well at an affordable price for mass manufacturing. They'd better keep at it, though, because thanks to the existing environmental reservoirs, there is a very real risk of a fourth plague pandemic. And I'm not just talking about the prairie dogs, or a select group of flea-carrying rodents. Over the years, scientists have learned Y. pestis can chill out in human body lice, soil, and amoebas in water! And for those of you with feline friends,
I should mention cats are susceptible to severe cases of plague. So if your cat keeps leaving you decapitated presents, maybe keep an eye out. It looks like this bacteria might plague us for a while longer. [♪OUTRO]
