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 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.

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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]