IN 1347 DEATH CAME to Europe. It ravaged for six years, and no one was spared. Old, young, rich and poor all died mysteriously. The mark of death was certain: Buboes would emerge in the groin or armpits, and within a few days, the victim would most likely die.

Most experts agree that around 25 million people died, approximately half of Europe’s population back then. Some have estimated that upwards of two thirds of all Europeans met an early demise.

The Black Death is known to most people, at least in Europe. It was bubonic plague, and it spread from somewhere in Central Asia, carried across Eurasia not by humans, but by rats, hitching rides on caravans and stowed away in the dark corners of ships. They utilized the great trade routes connecting India and Europe by ship, and China and Europe by the Silk Roads. The proto-globalization of the Middle Ages was the very foundation for the deadliest known disease in human history.

Or so the usual story goes.

But ever since the late 1980s, some historians and archaeologists have cast doubt on whether the Black Death had anything to do at all with plague. They cite various reasons for their skepticism, but the most common – and hardest to counter – is one glaring problem: the lack of rats.

But what is plague even?

First, a word about plague.

In short, it’s a bacterium that lives in rodents such as rats, beavers and squirrels. It’s not, as such, a human disease, but what we with a fancy word would call a zoonotic disease, meaning it’s an animal disease that can infect humans. That’s one of the reasons it’s so deadly: The bacterium isn’t geared to living in humans.

The plague bacterium jumps from rodents to humans by way of fleas. In the case of rats, they suck the blood from the infected rats, in turn getting infected themselves. And while plague isn’t necessarily deadly to rodents, rats are very susceptible to it. Soon, the infected rat population dies off. The fleas, infected with plague and bereft of their usual blood meal, starve and become desperate. They’ll turn to humans, trying to sate their hunger. But since the flea’s ability to actually digest its blood meal by then has been blocked by plague bacteria, now greatly multiplied, it’ll just infect its human victim without sating its own hunger. The flea then jumps to other people. In no time, one plague-ridden flea will have infected several humans.

What started as a rat epidemic has become a human epidemic.

Today, we have antibiotics that effectively treat plague. But in the Middle Ages, that invention was still several centuries off. Without effective treatment by modern medicine, plague is fatal in most cases.

Why some historians think the Black Death isn’t plague

Let’s return now to the absence of rats.

The first issue is that no contemporary source explicitly mention rats. This isn’t by itself proof that there weren’t significant rat populations. People will often neglect to mention things that are either commonsense to them or seems unimportant. It’s quite possible, then, that nobody suspected that rats were behind the disease. After all, medieval wisdom held that illness was either a product of a body out of balance with itself or foul-smelling air. The more religiously minded would see the disease as a punishment from God. And if those are your world views, you don’t implicate animals in the spread of disease.

Issue is, archaeologists haven’t managed to turn up rat skeletons in Scandinavia, a place we know was hit by the Black Death. And while sources might neglect to mention this or that, we should expect to find rat remains in the archaeological record since a rather sizeable population of rats is required to sustain a plague outbreak.

So are the sceptics right? Not necessarily.

The case for plague

Archaeologists have identified plague bacteria in several victims from the Black Death. (They subtract pulp from the teeth of the skeletons, which they then expose to DNA analysis. If plague bacteria are present, it means that the victim had plague – if not, well, it means there was no plague in that particular case.)

This isn’t to say that archaeology is foolproof. It’s unrealistic to analyze every single plague victim, so while we can see that plague was present in most cases, this does in fact only say something about the skeletons we have examined. It might hypothetically be the case that our results are simply biased, and the unanalyzed victims from the Black Death – a very big majority – did not have plague. This, however, is speculation. And highly unrealistic speculation at that.

What we know for a fact, however, is that plague was present in European populations at the time of the Black Death.

This doesn’t solve the lack of Scandinavian rats, though. If there were no rats to carry plague, then how did it ravage the frozen north of Europe?

One explanation is that it was carried north by other rodents and animals. Cats and squirrels routinely infect humans with plague, for example. But we’d still need a very sizeable cat or squirrel population in Scandinavia during the Middle Ages in close contact with people in order to lead to a plague epidemic. This, as far as I know, was not the case.

Another explanation can be found with the medieval skeletons that archaeologists have analyzed. For while a majority of them did have plague, not everyone did. And just because a plague epidemic is ravaging the land doesn’t mean that it’s the only killing disease.

Or maybe it’s just darn bad luck that we haven’t recovered medieval rat remains in Scandinavia. But with every excavation, that seems increasingly unlikely.

So what was the Black Death?

Well, it was a plague. That’s still the most convincing explanation that historians and archaeologists have come up with. But it’s quite feasible that it wasn’t just plague. After all, people die for a myriad of reasons, even during pandemics and epidemics. So while the skeptics haven’t overturned our understanding of the Black Death, they have questioned several assumptions and brought to light a lot of things we simply don’t know. These questions, however, are up to new generations of medieval historians to resolve.

Want to learn more?

The Black Death 1346-1353: The Complete History by Ole J. Benedictow (2004).

Deadly Companions: How Microbes Shaped Our History by Dorothy H. Crawford (2007).