Diseases Didn’t Just Shape History, They Control the Future

A new book explores the far-reaching impact of germs and viruses on human society.

You are horribly outnumbered.

Even within your own body, your 30 trillion human cells can’t compete with the 40 trillion or so bacteria that live rent-free in your gut, on your skin, under your toenails. 

Your very DNA owes a significant chunk—about 8 percent—of its content to retroviruses, which, when they infect a sperm or egg cell, can rewrite short sections of our genetic code in a way that’s passed down to the next generation. It’s thought that these snippets gave our distant ancestors the ability to form memories and carry their young in a womb instead of laying eggs—without them, humans could look very different.

And it doesn’t stop there. Even today, those bacteria living in your gut—your microbiome—may be influencing your behavior in ways that you can’t sense and that scientists don’t understand, releasing neurotransmitters to make you more sociable and more likely to spread bacteria, playing your brain like an instrument to serve their own ends.

It took the Covid-19 pandemic to really expose the power that germs have over our lives. But bacteria and viruses have been shaping our world in invisible ways for millennia, influencing not only our individual bodies but also the shape of the world we live in: history, politics, religion. That’s the argument made by public health researcher and sociologist Jonathan Kennedy in his compelling new book, Pathogenesis: How Germs Made History. “In the spring of 2020 loads of people were saying, ‘This is extraordinary, this is unprecedented,’” he says. “I had a pretty good idea that it wasn’t.”

Looking through the literature, Kennedy was struck by a question: “If bacteria and viruses had such a big impact on us as individuals, what impact have they had on us as aggregations of bodies: the body politic, the body economic, the body social?” In other words, how have germs influenced human history and, more pertinently—what kind of impact might a global pandemic have on what’s to come?

“Historians tend to see the natural world as a stage on which humans—sometimes great men, sometimes groups of people—act,” Kennedy says. “We have to change the conceptualization of history, we have to see ourselves as part of an ecosystem.”

That ecosystem can, Kennedy argues, help explain long-standing mysteries, like why Homo sapiens outlasted the Neanderthals, for instance—answer: a potent mixture of pathogens and interbreeding. It can also make sense of how small groups of conquistadors were able to overpower huge New World empires—infectious diseases like smallpox were transported across the Atlantic by the first arrivals, then decimated the New World populations so that by the time the conquests of Hernán Cortés and Francisco Pizarro began, once thriving communities had already been turned into ghost towns. “The population of the Americas fell by 90 percent in the century after Columbus arrived in Hispaniola,” Kennedy says. “The drop in population was so marked that you can still see it in ice cores that are drilled in Greenland. It had an impact on the temperature of the world.”

The success of the conquistadors has been attributed to guns, germs, and steel, but you could also point to a quirk of fate: The New World has fewer domesticable animals than the Old and fewer that live in large herds like cows and sheep. As a result, infectious diseases have had less chance to incubate and jump the species barrier to humans, and so people living in the Americas had never had the chance to build up immunity to pathogens like smallpox, which is thought to have jumped from livestock to humans in the early days of agriculture, around 10,000 BC. 

There are other compelling examples of germs changing the course of history: how the Black Death reduced the working population and made labor more valuable, sparking the end of feudalism; how malaria rendered much of Africa impenetrable until the 1880s until the widespread use of quinine (which comes from the bark of the South American cinchona tree).

Diseases may also be responsible for the spread of religions like Christianity, which exploded in popularity after the third-century Plague of Cyprian, which was, some scientists now believe, a type of hemorrhagic fever similar to Ebola. Christianity encouraged acts of kindness as a route into heaven—rather than fleeing from the sick, Christians nursed them back to health, drastically improving their survival rates. “Even with basic nursing, providing people with water and food, you could maybe save two-thirds of people who were sick,” Kennedy says. To the untrained eye, this would have looked a lot like a miracle—the best kind of publicity for any new religion. In comparison, “paganism didn’t provide a very helpful way for interpreting the impact of infectious disease outbreaks,” Kennedy says.

But the spread of Christianity also spread the notion of man’s dominion over nature. In the long term, that attitude has contributed to climate change and driven our relentless push into remote areas, both things that can distribute new diseases as we rub up against nature in strange new ways. 

Things came full circle with Covid, though. It remains to be seen what impact the latest pandemic will have. “Being in the eye of the storm it’s hard to tell, but if we look back at history there’s so many cases of pandemics, epidemics coming along, killing lots of people, harming societies, and creating the space for new ideas and new societies to emerge,” Kennedy says. “Probably when we look back at this period we’ll see that there were changes that were maybe already underway, but that Covid-19 has either accelerated them or changed the trajectory of history.”

Covid has perhaps already served as a humbling reminder of the natural order of things. “It’s been quite a shock to the way in which a lot of us see the power of humans,” Kennedy says. “You can make a pretty good argument that we have been living, and still live, and always will live in the age of microbes. Coming to terms with that is part of really learning how to live successfully on this planet.”

(Pathogenesis: How Germs Made History by Jonathan Kennedy is published on April 13.)

Diseases Didn’t Just Shape History, They Control the Future

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