National College of Physicians
National College of Physicians

The Germs That Transformed History

For eons, epidemics have caused mass deaths and social upheaval, with far-reaching effects on politics, trade, migration, colonization and conquest. The Covid-19 pandemic is an almost unique phenomenon in world history. The only precedent for its rapid spread to every continent, killing people everywhere and devastating both local economies and world trade, was the flu pandemic of 1918-19.

By Jared Diamond

The Covid-19 pandemic is an almost unique phenomenon in world history. The only precedent for its rapid spread to every continent, killing people everywhere and devastating both local economies and world trade, was the flu pandemic of 1918-19.

In both cases, the germs behind the pandemic weren’t especially lethal. Covid-19 and the flu both fall within the normal range of mild infectious diseases. Compared with smallpox and Ebola, they kill only a small percentage of their victims, and their person-to-person transmissibility isn’t unusual. What sets them apart—what has made them world-wide pandemics—is modern transportation: fast steamships for the flu, and now jet airplanes for Covid-19.

Of course, there have been a great many “mere” epidemics in human history, diseases that have spread more slowly over large areas, but their effects have been profound. Over the course of recorded history and now in the archaeological record, examples abound of germs producing high death tolls and social and political upheaval, with far-reaching effects on local economies, trade, migration, colonization and conquest.

Will Covid-19 transform our own era, too? Are we entering an age of pandemics? It is far too early to say, but the long history of germs as agents of historical change can provide needed perspective—and perhaps a window into how Covid-19 and its likely successors may shape our destiny, for good and ill.

Consider first a familiar case: the plague bacterium, which was transmitted from rodents to humans by fleas, and spread from Asia into Europe in the 14th century. The Black Death killed about a third of Europe’s population in the years 1347-51 and recurred with lower death tolls for many decades thereafter.

Its immediate effect on Western Europe’s economy and trade was disastrous. Paradoxically, though, its long-term effect was positive. By reducing the number of laborers, the Black Death forced landowners to start paying their tenants and to grant them more rights and freedoms. Societies became less rigidly stratified; nuclear families became stronger; and sanitation and quarantines developed to combat infectious diseases.

Europeans weren’t the plague’s only victims. In Central Asia, it had a big effect on history’s largest land empire, the Mongol Empire, which stretched from China to Persia and Russia. By cutting off travel and trade, plague severed connections between the empire’s parts, thereby causing Mongol control of Persia and China to collapse in 1335 and 1368, respectively, and control of Russia to wither away slowly.

Tropical diseases have had similarly profound effects. Only after quinine was found to be effective in treating malaria, for instance, could Europeans embark on their colonial conquests in New Guinea and Africa, carving up territories where European settlement had been impossible before.

Why, before quinine, were Europeans unable to establish themselves in New Guinea and Africa, where indigenous peoples had been thriving for hundreds of thousands of years? Because different peoples have different susceptibilities to the same disease—a major theme of medical history. Over thousands of years of exposure to malaria, New Guineans and Africans evolved many forms of genetic resistance through natural selection. Northern Europeans, by contrast, had no history of exposure to malaria, hadn’t evolved any of those genetic protections and hence were unlikely to survive long enough to develop antibodies.

The most lethal, permanent and far-reaching effect of germs in recorded history was to aid the European conquest and extensive replacement of Native Americans, Pacific Islanders and Aboriginal Australians. This was the reverse of what happened in Africa and New Guinea, where local diseases and differential susceptibilities enabled local peoples, for a time, to resist European invaders.

Perhaps the deadliest germ that Europeans brought with them was smallpox. The disease killed Europeans too, of course, but through natural selection over thousands of years of exposure, they had evolved partial genetic resistance. They also had acquired immunity by developing antibodies. In the centuries before vaccination was developed, smallpox was widespread in Europe, and almost everyone was exposed in childhood, when some died but most survived and remained immune thereafter.

When smallpox was brought by Europeans to the New World, all Native Americans of all ages lacked antibodies, and the mortality rate was high, especially among adults. In 1837, for example, when a steamship on the Missouri River accidentally introduced smallpox to a village of 2,000 previously unexposed Mandan Indians, all but 40 of the villagers died within a few weeks.

We often think of European settlers taking up arms to dispossess or kill Native Americans. In fact, most Native Americans died in bed of European diseases, leaving only a fraction to be defeated by guns on the battlefield. The most consequential of these outbreaks were the smallpox epidemics that struck the New World’s most powerful states, the empires of the Aztecs and Incas. The disease killed their emperors Cuitláhuac and Huayna Capac and millions of their subjects, making possible the conquests of Cortés and Pizarro.

But this familiar history raises a question: Why didn’t Native Americans, like Africans, have diseases of their own to which they had some genetic and acquired resistance—and which could have killed off invading Europeans?

The answer is relevant to understanding Covid-19 and other new diseases that have emerged over the past 40 years—AIDS, SARS, MERS, Ebola and Marburg. All of them are zoonoses, that is, diseases of animals. They reach humans from wild animals with which we came into close contact through hunting and animal markets.

We now know that, thousands of years ago, animals were also the source of our major epidemic infectious diseases: smallpox, measles, influenza, tuberculosis and others. The diseases infected us from domestic animals such as cattle, pigs and camels, with which we come into much closer, more frequent and more prolonged contact than with wild animals. It all began around 10,000 years ago, when people began to domesticate livestock.

Almost all of our livestock were domesticated in Eurasia, from Eurasian wild ancestors. The only large animal domesticated in the New World was the llama, which didn’t infect us because we don’t milk or cuddle llamas as we do cows and calves, or sheep and lambs. That is why the major epidemic diseases of history have all been Eurasian diseases. Compared with Eurasia, the Americas were a healthy environment, and Native Americans a healthy people, until Europeans arrived, bringing their livestock-derived diseases That is also why Native Americans lacked their own equivalent of smallpox to give back to Europeans.

As scientists have recently discovered, similar deadly encounters also happened long before written history could record them. Archaeologists have found abundant evidence of nomadic horse-mounted herders from the Asian steppes reaching Eastern Europe in prehistoric times. Those herders were the predecessors of the Huns, the Mongols and other steppe invaders who terrorized Europe over the past two millennia.

In recent years, geneticists analyzing ancient skeletons have found the DNA of steppe peoples appearing widely over Europe and partly replacing the DNA of Europe’s ancient farmers. But how could these horse-mounted nomads have successfully invaded Europe‘s forests, where their horses would have been of limited military value? Those same forests deprived the Huns and Mongols of their military advantage, so how could their ancient predecessors have succeeded where Attila and Genghis Khan failed?

Prehistoric European skeletons have revealed an agent of conquest more potent than horses: DNA of the bacterium causing plague. So perhaps steppe invaders didn’t kill ancient European farmers just with bows and arrows. A troop of plague-bearing nomads riding into a European farming community would have had an effect like that of a Missouri River steamship with a smallpox patient arriving at a Mandan Indian village in 1837.

This discovery may also contribute to answering the biggest unresolved question of historical linguistics: the wide distribution today of Indo-European languages, all closely related to each other, from India all the way west to Ireland, from Hindi and Russian to English and Spanish. Linguists and archaeologists studying the differences among these languages suggest that the Indo-European family expanded only within the last 10,000 years.

Biologically modern Homo sapiens has occupied all of Europe for at least the last 30,000 years, however, since the extinction of the Neanderthals. Surely, the modern humans who created the great cave paintings at Lascaux in France spoke fully modern human languages and didn’t just grunt. Yet the only non-Indo-European language remaining in Western Europe today is the Basque language of the Pyrenees.

One has to wonder whether languages related to Basque were the languages of Europe’s first farmers, most of whose languages were replaced by the Indo-European languages spoken by plague-infected steppe nomads around 5,000 years ago. If so, plague may help to explain why this article is published today in English, rather than in a language related to Basque.

Were germs also involved in some of the other great replacements of the prehistoric past? At least two prominent cases cry out for study.

First, as we know from their appearance, genes, languages and archaeological remains, the farming peoples now living in tropical Southeast Asia—Vietnamese, Thais, Malays, Indonesians, Filipinos and others—came originally from South China and Taiwan. But ancient skeletons show that, until around 3,000 B.C., the region was occupied by hunter-gatherers related to modern New Guineans and Aboriginal Australians.

Another intriguing case: Almost all modern peoples of subequatorial Africa today are blacks who speak Bantu languages, closely related to the languages of West African farmers. Again, though, all subequatorial African peoples until around 2,000 years ago were hunter-gatherers similar to the few surviving subequatorial hunter-gatherer groups of Pygmies and Khoisan peoples.

Both in Southeast Asia and in subequatorial Africa, archaeologists have assumed that denser populations and superior technology were what enabled Chinese or West African farmers to replace the indigenous hunter-gatherers. While that is undoubtedly part of the story, I’d suggest that the germs of the invading farmers and their domestic animals likely played a role too, infecting hunter-gatherers who had had no exposure to those diseases.

What do the histories of these early epidemics tell us about the pandemic that the world is now enduring?

One obvious conclusion is that Covid-19 is certain to have successors. Over the past 10,000 years, we humans acquired most of our infectious diseases from domestic animals. The main source of new diseases in recent decades has been large-scale contact with wild animals. China has closed its wild animal markets in response to Covid-19, but other points of contact remain: wild animal markets in other countries, the use of wild animals for traditional medicines and the African trade in bushmeat. As long as those routes remain open, we’re likely to see more diseases like Covid-19 in the near future.

And many of them will probably become pandemics because of jet planes and other forms of mass travel. Covid-19 has circled the globe in a matter of months; the Black Death of the Middle Ages lacked the means to spread so widely.

Finally, there is the issue of who is suffering most from Covid-19. The big killers of history and prehistory have involved differential mortality: peoples with some genetic and acquired protection from previous exposure, who then infected and caused mass mortality among previously unexposed peoples with no protection.

At first, it might seem that this doesn’t hold true for Covid-19. After all, the disease has existed for less than a year. No one had genetic protection or antibodies to protect them, which is why Covid-19 is killing people all around the world.

But as we all know by now, there is differential mortality with Covid-19; it just has nothing to do with pre-existing genetic resistance or antibodies. We’re seeing big differences in mortality between population groups in the U.S.—between poor Native Americans and African-Americans, for example, and other Americans—related to differences in pre-existing health conditions. We’re seeing even bigger differences among American states (lower mortalities in Montana than in New York) and among countries (much lower mortalities in New Zealand, Germany and Vietnam than in Italy and the U.S.) because of differences in geography and government policy. When Covid-19 reaches its peak in India and Africa, we are likely to see further disparities, related to differences in national wealth and health-care systems.

Covid-19 may thus increase inequality within and between countries. Inequality was causing big problems before the pandemic, of course, but now, thanks to modern travel and globalization, it means that poor, unhealthy, infected people will remain sources of re-infection for wealthy, healthy people.

A challenge for the future will be to ensure that face masks, vaccines, treatments and ventilators become available to everyone in the world. If they don’t become available, we shall all remain vulnerable. If they do become available, that global response to a global problem may inspire other global responses to other global problems. That would be a delayed benefit of the Covid-19 tragedy.

—Mr. Diamond, a professor of geography at the University of California, Los Angeles, is the author of “Upheaval: Turning Points for Nations in Crisis,” “Collapse: How Societies Choose to Fail or Succeed” and “Guns, Germs and Steel: The Fates of Human Societies,” for which he won the Pulitzer Prize.