In a flurry of wartime propaganda, American and European governments downplayed the epidemic, which helped it spread. Estimates of the final death toll range from 17 million to 100 million, depending on assumptions about the number of uncounted victims. Almost 700,000 people are thought to have died in the US – as a proportion of the population equivalent to more than two million people today.

Remarkably, the calamity left few visible traces in American culture. Writers Ernest Hemingway, William Faulkner and F. Scott Fitzgerald saw its terrible effects first-hand, but almost never mentioned it in their work. Nor did the flu affect US policies – Congress didn’t even allocate extra money for flu research afterwards.

Just a few decades after the pandemic, American-history textbooks by the distinguished likes of Arthur M. Schlesinger Jnr, Richard Hofstadter, Henry Steele Commager and Samuel Eliot Morison said not a word about it. The first history of the 1918 flu wasn’t published until 1976. Written by the late Alfred W. Crosby, the book is called America’s Forgotten Pandemic.

Americans may have forgotten the 1918 pandemic, but it did not forget them. Garthwaite matched NHIS respon­dents’ health conditions to the dates when their mothers were probably exposed to the flu. Mothers who got sick in the first months of pregnancy, he discovered, had babies who, 60 or 70 years later, were unusually likely to have diabetes; mothers afflicted at the end of pregnancy tended to bear children prone to kidney disease. The middle months were associated with heart disease.

Other studies showed different consequences. Children born during the pandemic grew into shorter, poorer, less educated adults with higher rates of physical disability than one would expect. Chances are that none of Garthwaite’s flu babies ever knew about the shadow the pandemic cast over their lives. But they were living testaments to a brutal truth: pandemics – even forgotten ones – have long-term, power­ful after-effects.

The distinguished historians can be forgiven for passing over this truth. Most modern people assume that our species controls its own destiny. “We’re in charge!” we think. Being modern people, historians have had trouble, as a profession, accepting that brainless packets of RNA and DNA can capsize humankind in a few weeks or months.

The convulsive social changes of the 1920s – the frenzy of financial speculation, the resurgence of the Ku Klux Klan, the explosion of Dionysian popular culture (jazz, flappers, speakeasies) – were easily attributed to the war, an initiative directed and conducted by humans, rather than to the blind actions of microorganisms. But the micro­organisms likely killed more people than the war did. Their effects spread across the globe, emptying city streets and filling cemeteries on six continents.

Unlike the war, the flu was incomprehensible – the virus wasn’t even identified until 1931. It inspired fear of immi­grants and foreigners, and anger towards the politicians who played down the virus. Like the war, influenza (and tuberculosis, which subsequently hit many flu sufferers) killed more men than women, skewing sex ratios for years afterwards. Can one be sure that the ensuing, abrupt changes in gender roles had nothing to do with the virus?

We will probably never disentangle the war and the flu. But one way to summarise the impact of the pandemic is to say that its magnitude was in the same neighbourhood as that of the “war to end all wars”.

Consider the Black Death. Sweeping through Europe from about 1347 to 1350, the plague killed somewhere between a third and half of all Europeans. In England, so many people died that the population didn’t climb back to its pre-plague level for almost 400 years.

With the supply of workers suddenly reduced and the demand for labour relatively unchanged, medieval land­owners found themselves in a pickle: they could leave their grain to rot in the fields, or they could abandon all sense of right and wrong and raise wages enough to attract scarce workers.

In northern Italy, landlords tended to raise wages, which fostered the development of a middle class. In southern Italy, the nobility enacted decrees to prevent peasants from leaving to take better offers. Some historians date the separation in fortunes of the two halves of Italy – the rich north, the poor south – to these decisions.

When the Black Death began, the English Plantagenets were waging a long, brutal campaign to conquer France. The population losses meant such a rise in the cost of infantrymen that the whole enterprise foundered. English nobles did not occupy French chateaux. Instead they stayed home and tried to force their farmhands to accept lower wages. The result, the Peasants’ Revolt of 1381, nearly toppled the English crown. King Richard II narrowly won out, but the monarchy’s ability to impose taxes, and thus its will, was permanently weakened.

When I went to college, in the 1970s, pre-med students carried around a fat textbook co-written by the Nobel Prize-winning virologist Macfarlane Burnet. “The most likely forecast about the future of infectious disease,” it sunnily concluded, “is that it will be very dull.”

Such optimism was not exceptional. A few years later, Robert Petersdorf, a future president of the Association of American Medical Colleges, contemplated the current crop of MDs seeking certification in infectious disease and said, “I cannot conceive of a need for 309 more infectious-disease experts unless they spend their time culturing each other.”

But not many non-scientists took these warnings to heart. The public has not enjoyed its surprise re-entry into the world of contagion and quarantine – and this unhappiness seems likely to have consequences.

Scholars have long posited that the shattering of norms by the Black Death was the first step on the path that led to the Renaissance and the Reformation. Neither government nor Church could explain the plague or provide a cure, the theory goes, leading to a crisis in belief. Secular and religi­ous leaders died just like common people – the Black Death killed the archbishop of Canterbury, Thomas Bradwardine, a mere 40 days after he assumed office. People sought new sources of authority, finding them through direct personal experience with the world and with God.

To some extent, all of this is surely true. The plague came in waves, and after each wave doctors, clerics and chroni­clers speculated about the causes and described the treatments they had seen deployed.

As the University of Glasgow historian Samuel Cohn Jnr has shown, the early claims about the plague’s origin invoked “floods of snakes and toads, snows that melted mountains, black smoke, venomous fumes, deafening thunder, lightning bolts, hailstones and eight-legged worms that killed with their stench”. Some writers blamed the poor: their fecund­ity, their improvidence, their sinfulness. Others pointed fingers at that ever-ready European bogeyman, the Jew.

Scared Europeans sought favour from the heavens, most famously taking off their clothes in groups and striking one another with whips and sticks. Images of half-nude flagellants have, since Monty Python, become a comic staple. Far less comical was the accompanying flood of anti-Semitic violence. As it spread through Germany, Switzerland, France, Spain and the Low Countries, it left behind a trail of beaten cadavers and burned homes.

Within a few decades, Cohn wrote, hysteria gave way to sober observation. Medical tracts stopped referring to conjunctions of Saturn and prescribed more earthly cures: ointments, herbs, methods for lancing boils. Even priestly writings focused on the empirical. “God was not mentioned,” Cohn noted. The massacres of Jews mostly stopped.

It’s easy to see this as a comforting parable of rationality winning out over the engines of rumour, prejudice and superstition, ultimately leading to the Renaissance and the Enlightenment. But the lesson seems more that humans confronting unexpected disaster engage in a contest for explanation – and the outcome can have consequences that ripple for decades or centuries.

Christopher Columbus’ journey to the Americas set off the worst demographic catastrophe in history. The indigenous societies of the Americas had few diseases – no smallpox, no measles, no cholera, no typhoid, no malaria, no bubonic plague. When Europeans imported these diseases, it was as if all the suffering and death these ailments had caused in Europe during the previous millennia were compressed into 150 years.

Up to nine-tenths of people in the Americas died. Many later European settlers believed they were coming to a vacant wilderness. But the land was not empty; it had been emptied – a world of loss encompassed in a shift of tense.

Absent the diseases, it is difficult to imagine how small groups of poorly equipped Europeans could have survived in the alien ecosystems of the Americas. “I fully support banning travel from Europe to prevent the spread of infectious disease,” the Cherokee journalist Rebecca Nagle remarked after Trump announced his plan to do this. “I just think it’s 528 years too late.”

For Native Americans, the epidemic era lasted for centuries. Isolated Hawaii had almost no bacterial or viral disease until 1778, when the islands were “discovered” by British explorer James Cook. Islanders learned the cruel facts of contagion so rapidly that by 1806, local leaders were refusing to allow European ships to dock if they had sick people on board.

Nonetheless, Hawaii’s king and queen, Kamehameha II and Kamamalu, travelled from their clean islands to London, that cesspool of disease, arriving in May 1824. By July they were dead – measles.

The royals had gone to Britain to negotiate an alliance against the US, which they correctly believed had designs on their nation. The monarch’s successor, 12-year-old King Kamehameha III, could not resume the talks. The results changed the islands’ destiny. Undeterred by the British Navy, the US annexed Hawaii in 1898. Historians have seldom noted the connection between measles and the presidency of Obama.

As a rule, epidemics create what researchers call a “U-shaped curve” of mortality – high death rates among the very young and very old, lower rates among working-age adults. (The 1918 flu was an exception; a disproportionate number of twenty-somethings perished.) For Native peoples, the U-shaped curve was as devastating as the sheer loss of life.

As an indigenous archaeologist once put it to me, the epidemics simultaneously robbed his nation of its future and its past: the former, by killing all the children; the latter, by killing all the elders, who were its storehouses of wisdom and experience.

So far, the evidence suggests: not much. The speed with which pundits in the US emerged to propose that it could more easily tolerate a raft of dead oldsters than an econo­mic contraction indicates that the reservoir of appreciation for today’s elders is not as deep as it once was. This change may reflect another: today’s old are older than the old of the past, when lifespans were shorter.

Past societies mourned the loss of collective memory caused by epidemics. Ours may not, at least at first.

These measures may suggest a community in the grip of fear. But the masks and signs and gloves seem more like the “victory gardens” outside homes during World War II – cheerful public notices of people doing their part. Most important, Hong Kong may have contained Covid-19 faster than any other place in the world.

Inside was a cross section of Hong Kong citizens – young and old, trainers and salaryman shoes, quite a few in makeshift masks. I thanked the proprietor for rescuing me from what could have been an unpleasant encounter. “We are all here together,” someone said.

Later it occurred to me that a possible legacy of Hong Kong’s success with Sars is that its citizens seem to put more faith in collective action than they used to. I’ve met plenty of Hongkongers who believe that the members of their community can work together for the greater good – as they did in suppressing Sars and will, with luck, keep doing with Covid-19. It’s probably naive of me to hope that containing the coronavirus would impart some of the same faith elsewhere, but I do anyway.

Financial support for this article was provided by the HHMI Department of Science Education.

Text: The Atlantic Magazine