How Germs Shaped History

Humans fill our history pages with tales of grand strategies, pivotal battles, and deliberate choices of leaders. But we are often just the playthings of viruses and bacteria.

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Humans have a stubborn tendency to put themselves at the center of the universe. For much of our history, we imagined that Earth was the focal point of the universe and that humanity was the pinnacle of life—above, rather than part of, nature. Since then, we’ve had some pretty profound scientific wake-up calls, realizing that we live in a world that rotates around the sun, in one of countless galaxies, and that humans are, like every other species on the planet, merely one chain-link species in an unbroken evolutionary history.

But we nonetheless continue to explain our collective history with ourselves in the spotlight. Our history books are filled with tales of pivotal choices by powerful people, technological revolutions, and epic battles. That has led us to systematically ignore a crucial factor that has shaped history more than any individual: microbes that can make us sick. We are, it turns out, merely the transient playthings of bacteria and viruses, which have shaped humanity far more than we realize.

An Indonesian volcano helped spark the Black Death, which ravaged humanity for centuries, eventually paving the way for the Protestant Reformation. The American Revolution was won, partly, thanks to British soldiers’ lack of immunity to pathogens in the southern colonies. Scotland is part of the United Kingdom today partly due to a debt incurred after an expensive expedition was decimated by disease. And the modern welfare state grew out of a response to the devastating toll of infectious illnesses spurred by the Industrial Revolution.

So argues Jonathan Kennedy in an excellent new book called Pathogenesis. In it, Kennedy makes a convincing case that we misunderstand history because we focus exclusively on human choices and social trends, rather than exploring how our social trajectories have been repeatedly changed in the most astonishing ways by invisible bacteria and viruses.

This is certainly true, both across vast stretches of geological time, and in the more recent sagas that we normally think of as history. Our species and our modern world were both profoundly shaped by germs.

Why Don’t Humans Lay Eggs?

As I’ve recently written in an edition called “The Greatest Fluke of All-Time,” complex life was only made possible when a bacterium merged with another single-celled organism.

But Kennedy brings in another arresting discovery: that live birth evolved after a shrew-like creature was infected by a virus more than a hundred million years ago. As he writes: “The scientists concluded that a crucial function of the placenta didn’t emerge gradually as a result of evolution by natural selection but was suddenly acquired when a retrovirus inserted its DNA into our ancestor’s genome.” But for a virus in the distant past, perhaps humans would still be having gender reveal parties, timed for just before the hatching.

Once modern humans emerged, evolution by natural selection changed how our species developed over time. In our high school textbooks and pop evolution non-fiction, we are told that selection pressures led to a “survival of the fittest.” Often, we tend to think of evolutionary fitness as tied to traits like social intelligence, physical strength, or the ability to work with others to stave off predators or rival tribes. But, Kennedy notes, that probably wasn’t actually the strongest factor for much of human history. Instead, what “the fittest” meant was often tied to immune systems, as humanity faced death from parasites and bacteria.

Malaria, which has now been relegated to the poorest parts of the world, killed as many as 300 million people in the 20th century. But the first evidence of malaria dates back thirty million years, discovered in ancient mosquitoes preserved in amber. For the entire time Homo sapiens have graced the planet, these deadly parasites have been quietly shaping history, from prehistory to the present.

Herodotus explains that those who built the Egyptian pyramids were given garlic to help protect them from malaria (recent studies have shown that garlic oil does help inhibit malaria). Homer mentions malaria in The Iliad. And an ancient Chinese medical text blamed the symptoms of malaria on “three demons—one carrying a hammer, another a pail of water, and the third a stove.”

The Black Death and the Army Killers

Around 12,000 years ago, when humans settled down and became overwhelmingly agricultural, our relationship with disease changed. Suddenly, infectious diseases that spread rapidly across a sedentary population became possible. And, as we began to live alongside other mammals and birds (which had their own pathogens to worry about), humans began getting infected by a wider profile of microbes, which made the leap from animals to us.

In the Medieval period, Kennedy cites research that suggests the Black Death—which researchers now believe was transported by gerbils—may have been triggered by a climactic shift produced by a major volcanic eruption in Indonesia. As countries were ravaged by plague, millions turned to the Church for answers, only to find that many clergy had simply fled their posts, trying to escape with their lives. This, Kennedy suggests, created widespread anger, conditions that made Europe ripe for a religious revolution—which culminated in the Reformation.

Some of these stories are a bit too neat, with a conception of history that is slightly too monocausal. (The causal chain that produced the Reformation had countless links, some obvious, others hidden, not just a few tidy ones that can be easily pinpointed. History is far more accidental, random, and contingent than we are led to believe, and our narratives almost always offer just a sliver of the “real” causal explanation).

But I’m persuaded by the viewpoint that we have wrongly discounted how much disease has shaped our societies and even our individual lives. After all, I completed my DPhil at New College, Oxford, which was established by a rich bishop in 1379 as a way to train replacement monks and clergymen for Winchester Cathedral, after as many of half of them perished in the Black Death of 1348-1350.

Pathogenesis also highlights how our battle with infectious diseases affects history. For example, there’s extensive discussion of the role of malaria on cutting down British soldiers sent to the southern colonies during the American Revolution. And while we think of the devastating toll of battles like Gettysburg and Antietam when we try to understand turning points in the US Civil War, an estimated two-thirds of the 660,000 deaths during the conflict came from disease, not from enemy muskets.

Cromwell and the Early Skeptics of Modern Medicine

One detail from Kennedy’s book that stood out is that in our battle with infectious disease, people throughout history have been like today’s anti-vaxxers: stubbornly resistant, for irrational reasons, to new-fangled medicines that might just save their life.

Oliver Cromwell, the scourge of the monarchy during the English Civil War, came down with malaria in 1658. Catholic Jesuits had already discovered that quinine, “found in the bark of cinchona trees that grow in the eastern foothills of the Andes,” could help those struck down with malaria to survive. Cromwell, an ardent Protestant who was aware that the cure had been discovered by Catholics, refused to take it. He died. His successor, Charles II, also got malaria. He happily gulped down some quinine and survived. (Of course, it is also more widely known that Gin and Tonic cocktails were invented to help British colonialists survive malaria in India).

Kennedy also covers more familiar ground, such as the role of disease in colonial conquests and the ways in which the Industrial Revolution produced a field day for germs, as humans packed themselves together in squalor. And he covers Mao’s Four Pests campaign, in which an attempt to exterminate rats, flies, mosquitoes, and sparrows inadvertently led to mass famine and millions of deaths (a story I previously covered in my podcast, Power Corrupts, for an episode called “Catastrophic Miscalculations").

My only gripe with Pathogenesis is that, for some reason, several chapters or sub-sections start with either a reference to The Lord of the Rings, Game of Thrones, or Monty Python. These would be better left on the editor’s cutting floor; the real stories are gripping enough—no need to bring in fantasy or comedy sketches when real-world history is so fascinating. But this is a minor criticism.

Pathogenesis is a wonderful book, erudite and eminently readable. It reminds us of a profound truth: history is not an endeavor that is sole authored by humans. Rather, we are the by-product of millions of years of strange, complex dynamics on this astonishing planet we call home. And, as we too often forget, microbes were around long before us, and they’ll be around long after we’re all dead, too. We’re just making a cameo, and more often than not, they, not us, are directing how history unfolds.

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Recommended Viewing and Reading

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