Exactly one year ago, on January 11, the first death from a confirmed case of Covid-19 was reported in China.
Today, the US is inching toward 400,000 confirmed deaths, the world toward 2 million. With alarming milestone after alarming milestone, the temptation grows to compare Covid-19 to the other horrific pandemics of history. It is not as bad, of course, as the 1918 flu pandemic, or as the notorious Black Death. But it has long surpassed the death toll of SARS (2002-2004), MERS (2012, 2015, and 2018), the 1957-58 and 1968-1970 pandemic influenzas, and the 2009 swine flu. By sheer number of casualties, Covid-19 ranks among the 10 deadliest plagues in history.
Some have taken a stab at putting the pandemic in historical context. At the start of the Covid-19 pandemic, the Washington Post visualized the devastating plagues of history, with Covid-19 by comparison a tiny dot. (By today, tragically, it’d be a much bigger dot.) This vivid graphic ranks Covid-19 the ninth deadliest in history
Those rankings should come with caveats. Our methods for measuring the deadliness of plagues past aren’t very good, and there’s a lot of guesswork involved. Our methods for counting the current death toll of Covid-19 have some serious flaws as well. Excess death measures suggest that the official toll is too low, even in countries with fairly good testing and death reporting like the US, and other countries do much less testing and death reporting.
Moreover, while Covid-19’s overall global death toll is among the 10 highest in history, that mostly reflects that the world’s population has grown a lot. The Black Death, which devastated Europe and killed around 30 percent of everyone alive on the continent, likely killed between tens of millions and a hundred million people, while a plague as deadly today, if it spread around the whole world, would kill more than 2 billion.
With those caveats in mind, there is a lot that we can learn from stacking Covid-19 up against history’s most notable plagues. Some of the things that stand out are ways that we got lucky. Most of the plagues that ravaged humanity in previous centuries particularly affected young children and babies, while the coronavirus has mostly spared them. Infection with the coronavirus seems to confer long-lasting immunity, and the virus doesn’t mutate too quickly; we could have been afflicted with a virus that is harder for our immune system to respond to, or that changes quickly enough that our vaccines aren’t effective against variants.
Other lessons from past plagues are reminders of the long road ahead. For example, many of these pandemics lasted years and years, and then became endemic, with recurrences for centuries afterwards. The Black Death, for example, was merely one big recurrence of a disease that had stalked humanity for hundreds of years at that point, and that was a major danger all the way through the invention of antibiotics.
By far the most notable takeaway is that, for most of our history, humanity has been relentlessly stalked by infectious disease. Devastating plagues ravaged major European cities every 20 years, if not more often, through the 1500s and 1600s. Half of children died of disease before they reached adulthood.
Modern medical knowledge and relentless vaccination programs made a world ruled by infectious disease feel like a thing of the past — until it suddenly wasn’t.
But even in this, our plague year, the modern world has seen far less risk of death from infectious disease than people saw in an average year in the premodern world. The coronavirus is a reminder we haven’t entirely triumphed over disease — but, put in its historical context, it’s also a reminder that things used to be much, much worse.
Let’s be clear about this: Ranking all the plagues in human history is a nearly impossible task.
The historical record is often very scant, and some of the worst plagues in history — like the smallpox that swept the Americas after contact with Europe — occurred mostly in societies without written birth or death records, leaving historians stuck guessing at their overall impact.
And making comparisons across pandemics is tricky: The absolute number of deaths from a disease is, of course, the result of both how deadly the disease is and how many people there are. In 1300, the century of the Black Death, there were around 400 million people in the world. By 1918, when the Spanish flu hit, there were nearly 2 billion.
That said, there is enough information for us to compare the worst plagues in history, and there is consensus on the worst of the worst.
At the top, of course, is the Black Death, an outbreak of bubonic plague that spread across much of Asia, Europe, and North Africa in the mid-1300s. The bubonic plague is spread by a bacterium carried in rats and fleas, called Yersinia pestis. Today, it is easily treated with antibiotics, but before they were developed, half of people infected might not survive. The victims would become feverish and achey, while lymph nodes swelled horrifyingly, sometime to the size of a chicken egg — called “buboes,” and the source of the name of the disease. While rats and fleas primarily transmitted it, when it became pneumoniatic (infecting the lungs) it could also spread directly person-to-person.
The Black Plague’s death toll is fiercely debated, with many historians estimating that between 25 million and 200 million people died in the space of five years. That’s a range of 5 percent to 40 percent of the world’s population at the time. For context, confirmed deaths from Covid-19 account for .0025 percent of the world’s population.
A contender for the next most devastating outbreak, the mid-500s Plague of Justinian, was also caused by the bubonic plague. Over the space of about two years, it afflicted the Byzantine Empire and from there, much of Europe. But our records of events from this time period are exceptionally shaky. Some historians believe it represented the first bubonic plague outbreak in history, and they estimate its death toll in the tens of millions. Other historians have pushed back, arguing that later scholarship magnified the plague and that deaths ought to be presumed to be considerably lower (more on this later).
While the various outbreaks of bubonic plague are the deadliest plagues on the list per-capita, they are not actually the ones that have killed the most people. In sheer numbers killed, the Black Death is possibly surpassed by the 50 million deaths worldwide in the 1918-1919 flu outbreak known in the US as the Spanish flu (though it didn’t originate in Spain — it acquired the name because, as the country was neutral in World War I, its toll was more accurately reported there than elsewhere in Europe).
The Spanish flu was an influenza virus much like the ones that circulate our world every year, but much deadlier. It had one particularly devastating quirk. Almost all influenzas are deadliest to the elderly and the very young, but the Spanish flu was exceptionally deadly to young adults. A hundred years later, historians are still theorizing about why that was the case. The Spanish flu mostly killed people through pneumonia — the flu weakened the lungs, after which secondary infections would arrive.
The other devastating killer pandemic of the 20th century — HIV/AIDS, which is estimated to have killed around 35 million people — is quite different from the other entries on the list. While most of them are fast-moving, contagious, or insect-transmitted diseases that swept the world in the space of a few years, HIV/AIDS, which is transmitted only through bodily fluids, spread slowly over the course of decades, abetted by the indifference of politicians who thought at first that the virus only affected gay men.
While HIV/AIDS used to be nearly 100 percent fatal, good treatments now exist, and the virus claims the most lives in poor countries where those treatments are not widely available despite their critical importance. Almost a million people still die every year of HIV/AIDS.
Other pandemics that rank among the worst in history include several influenza outbreaks, including the 1890 flu, the 1956-1958 flu, and the 1968 flu. As the study of viruses was just beginning in 1890, that outbreak’s origins are unknown.
What makes some flu seasons so much deadlier than others? We can identify specific deadly features of each of these viruses, but we don’t know for sure what made them emerge in those years and not others — and many public health officials worry a future influenza outbreak could be that bad or worse.
Covid-19, of course, has now been certified as the cause of death for 1.94 million people in the space of one year. That makes it worse in absolute terms than most influenza pandemics in history, except 1918’s; worse than the seven cholera pandemics of the 19th and early 20th century; but much less bad than HIV, 1918, or the Black Death and associated bubonic plague outbreaks. Every mentioned pandemic surpasses it in terms of deaths per capita, though — much of its high death toll is a consequence of the world’s population having grown from about 2 billion in 1920 to more than 7.8 billion today.
The challenge of ranking plagues
Looking closely at the trouble we have had in estimating deaths from the coronavirus this year will make it obvious why historical plague ranking is so imprecise and involves so much guesswork.
How many people have died so far of Covid-19? Official statistics maintained by the Johns Hopkins Center for Systems Science and Engineering have a precise answer: about 1.94 million. But that is the number of deaths following confirmed cases of Covid-19 — and many people die of Covid-19 without ever getting formal confirmation that is what they had.
Instead, most epidemiologists prefer to estimate the total toll of a disease with a different approach: excess mortality. Simply put, we measure how many people died this year. Then we compare that to an estimate of how many people would have died this year under normal conditions — that is, if there hadn’t been a global pandemic.
This approach isn’t perfect. For example, deaths in car accidents are down while many commutes and vacations are cancelled. Other infectious diseases have been suppressed by the public health measures taken against Covid-19. That has decreased mortality, which will cause us to underestimate Covid-19 deaths if we look at total deaths.
Nonetheless, excess mortality is often the best tool historians have to estimate the toll of pandemics. Here is an excess mortality chart showing excess mortality in the US in 2020:
In general, measuring excess mortality finds that the toll of Covid-19 has been larger than officially reported — even in rich countries like the US. One December study in the Annals of Internal Medicine estimated that from March to August, 220,000 excess deaths attributable to Covid-19 occurred in the US, compared to 169,000 confirmed deaths during that period.
But there isn’t accurately reported excess mortality data for most of the world. Often, there isn’t even accurate data on deaths during previous years that could be effectively used to establish a baseline for excess mortality measurement.
And this problem, while thorny for evaluating the Covid-19 numbers, is especially difficult for most of the other pandemics on the list. Above, I mentioned that the Black Death is thought to have killed as many as 200 million people. But its actual toll is hotly contested by historians. For the Justinian Plague, the situation is even worse. We rely mostly on limited surviving records from a handful of individuals, including the Byzantine court historian Procopius of Caesarea. Some historians estimate its toll might be as high as 100 million dead.
But that’s probably wrong, historian Lee Mordechai and other authors argued in an article last year declaring the Justinian Plague overblown. “The existing direct evidence consists of several historical narrative texts and 2 inscriptions,” they argue — not enough evidence to make claims that tens of millions of people died.
It takes a lot of historical background even to dig into some of the disagreements surrounding the toll of the Justinian Plague or the Black Death. Certainly lots of people died. But do we really know enough to rank historical plagues from least to most devastating, as is so perpetually tempting?
The responsible way to do it is probably with enormous error bars. The Justinian Plague, such a chart might read, killed between 2 million and 100 million people. That’s extraordinary uncertainty — but it’s what we are often working with, when trying to estimate health data from antiquity.
And even the several historical texts and inscriptions are more than historians have to go off when estimating the ravages of smallpox, measles, and other diseases spread by European contact in parts of the Americas that did not have written records. Some believe that up to 80-90 percent of native Americans died; with few records, though, the population of the continent pre-disaster is difficult to estimate.
Historians sometimes rely on DNA analysis, which finds a population bottleneck at the time of contact, estimating that the population of women of reproductive age fell 50 percent. There would likely have been higher mortality in other age groups.
Today’s horrors in perspective
Given all this uncertainty, is there even any value to comparing plagues across history? I do think there are a few crucial takeaways that are not diminished by the uncertainty inherent in such a historical exercise.
The first is that it helps us grasp that diseases can get much worse than the one we are currently facing. Covid-19 has devastated our world, but there are a few blessings: it very rarely strikes children, and its infection fatality rate — the percentage of those who are infected who die — is much lower than for many other famous plagues. Epidemic diseases like smallpox frequently killed 30 percent of those infected. Bubonic plague killed up to half of those it sickened. Both of them were particularly deadly for children.
Another reason such an exercise is valuable is that it can be a crucial reminder that this has happened before and, unless we get spectacularly good at halting outbreaks in their tracks, it will happen again. In an increasingly interconnected world, there will be a next pandemic — whether caused by another SARS-family coronavirus, by an influenza, or by an engineered disease. As the global population nears 8 billion, it would not take a disease anywhere near as deadly as the untreated bubonic plague to become the deadliest outbreak in history.
And yet somehow humanity persists. The bubonic plague is, of course, now treatable. HIV, still a driver of more than 1 percent of all global deaths annually, is not a death sentence anymore with appropriate treatment. Smallpox, the likeliest culprit in the mass deaths in the Americas after European contact, has been eradicated.
The point is: Disease isn’t inevitable. While many countries grapple with spiking coronavirus cases, others that took the disease seriously from the beginning still have remarkably few infections. And better tools for disease surveillance and rapid response might help catch the next coronavirus pandemic at the root.
Nothing can devastate the world like a plague, but nothing can fight a plague like humans can.