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Kyle Harper’s Plagues Upon the Earth is a remarkable accomplishment that weaves together microbiology, history, and economics to understand the role of diseases in shaping human history. Harper, an established historian known for his first three books on Rome and late antiquity, has an impressive command of virology, bacteriology, and parasitology as well as history and economics. In “Plagues Upon the Earth.” he explains all of these clearly and with many arresting turns of phrase and insights:
There are about seventy-three bacteria among major human pathogens–out of maybe a trillion bacterial species on earth. To imagine bacteria primarily as pathogens is about as fair as thinking of human beings as mostly serial killers.
Despite the tingling fear we still feel in the face of large animals, fire made predators a negligible factor in human population dynamics. The warmth, security and mystic peace you feel around the campfire has been instilled by almost two million years of evolutionary advantage given to us by the flames.
Mosquitos are vampires with wings….The blood heist itself is an amazing feat. Following contrails of carbon dioxide that lead to her mark, the female mosquito lands and starts probing. Once she reaches her target, she inserts her tube-like needle, as flexible as a plumber’s snake, into the skin. She pokes a dozen or more times until she hits her mark. The proboscis itself is moistened with compounds that anesthetize the victim’s skin and deter coagulation. For a tense minute or two, she pulls blood into her gut, taking on several times her own weight, as much as she can carry and still fly. She has stolen a valuable liquid full of energy and free metals. Engorged, she unsteadily makes her getaway, desperate for the nearest vertical plane to land and recuperate, as her body digests the meal and keeps only what is needful for her precious eggs.
What I like best about Plagues Upon the Earth is that Harper thinks like an economist. I mean this in two senses. First, his chapters on the Wealth and Health of Nations and Disease and Global Divergence are alone worth the price of admission. In these chapters, Harper brings disease to the fore to understand why some nations are rich and others poor but he is well aware of all the other explanations and weaves the story together with expertise.
The second sense in which Harper thinks like an economist is deeper and more important. He has a model of parasites and their interactions with human beings. That model, of course, is the evolutionary model. To a parasite, human beings are a desirable host:
Just as robbers steal from banks because that is where the money is, parasites exploit human bodies because there are high rewards for being able to do so…. for a parasite, there is now more incentive to exploit humans than ever…look at human energy consumption…in a developed society today, every individual consumer is the rough ecological equivalent to a herd of gazelles.
That parasites are driven by “incentives” would seem to be nearly self-evident but in the hands of a master simple models can lead to surprising hypothesis and conclusions. Harper is the Gary Becker of parasite modeling. Here’s a simple example: human beings have changed their environments tremendously in the past several hundred years but that change in human environment created new incentives and constraints on parasites. Thus, it’s not surprising that most human parasites are new parasites. Chimpanzee parasites today are about the same as those that exploited chimpanzees 100,000 years ago but human parasites are entirely different. Indeed, because the human-host environment has changed, our parasites are more novel, narrow, and nasty than parasites attacking other species.
It’s commonly suggested that one of the reasons we are encountering novel parasites is due to our disruptions of natural ecosystems, venturing into territories previously unexplored by humans, thereby releasing ancient parasites that have lain dormant for millennia. Like the alleged curse of Tutankhamun’s tomb we are unleashing ancient foes! Similarly, concerns are voiced that climate change, through its effect on permafrost melt, may liberate “zombie” parasites poised for retaliation. But ancient parasites are not fit for human hosts as they have not evolved within the context of the contemporary human environment. So, while I don’t trivialize the potential consequences of melting permafrost, I think we should fear much more relatively recent diseases such as measles, cholera, polio, Ebola, AIDS, Zika, and COVID-19. Not to mention whatever entirely new disease evolution is bound to throw in our path.
Indeed, one of the most interesting speculation’s in Plagues Upon the Earth is that “global divergences in health may have reached their maxima in the early twentieth century.” The reason is that urbanization and transportation turned the new diseases of the industrial era, like cholera, tuberculosis and the plague (the latter older diseases but ideally primed for the industrial era) into pandemics (also a relatively recent word) at a time when only a minority of the world had the tools to combat the new diseases.
Science, of course, is giving us greater understanding and control of nature but our very success increases the incentives of parasites to breach our defenses.
[Thus,] the narrative is not one of unbroken progress, but one of countervailing pressures between the negative health feedback of growth and humanity’s rapidly expanding but highly unequal capacities to control threats to our health.
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