Plagues upon the Earth Disease and the Course of Human History

It might sound crass to write that the COVID-19 pandemic is just the latest in a long line of infectious disease outbreaks, but a little perspective helps. Historian Kyle Harper previously impressed me with his study on the role of climate and disease in the decline of the Roman Empire. In Plagues Upon the Earth, he offers a global, multidisciplinary environmental history of infectious disease, showing that it is a force that has both shaped and been shaped by human history. This magnificent book stood out as much for its nuance and academic rigour as it did for its readability.

There is currently no shortage of big history books on pandemics. What makes Plagues Upon the Earth stand out? Briefly, it is so much more than just a potted history of "celebrity" diseases such as bubonic plague, smallpox, or measles. In particular, Harper carefully revisits existing narratives in light of new data and methods. Although I can only touch on a fraction of what impressed me, what follows are seven reasons why I think this book is worth your time.

First, it is appropriately evolution-centric, with the first two chapters focusing on microbiology, primatology, and especially evolutionary biology. His discussion of horizontal gene transfer, the mind-blowing concept of viral quasispecies, or the relentless nature of evolution shows he has read the relevant literature and can explain why viruses and microbes outwit us time and again.

Second, and equally appropriate, the book is germ-centric: "it's a microbe's world. We're just living in it" (p. 20). From their perspective, we are "warm, nutrient-rich, well-defended host environments" (p. 43) that, incidentally, also took over the world. A recurrent theme is that disease ecology shapes us as much as we shape it. Harper warns that "we should not treat the biology of disease like a kind of ghost in the machine, beyond all explanation" (p. 333). Two examples will have to suffice. We have identified 5000-year-old bubonic plague victims, but pandemics only became inevitable later, after rats had spread globally in the wake of trade, urbanization, farming, and grain storage. Harper furthermore details the human factors that contributed to the general crisis of the 17th century.

Third, Harper pays particular attention to new studies on phylogenetics, genomics, and ancient DNA. Does this kind of data have a special status? I would argue so: compared to historical records, DNA is eyewitness testimony without an ideological axe to grind. As with The Fate of Rome, Harper is not out to overthrow established narratives but to carefully reexamine them to see what genetics can add. The following are but a few examples. Agriculture seems to have spread by farmers pushing out hunter-gatherers, rather than the latter adopting farming practices. Morbiliviruses jumped from rodents and bats via cattle to humans. So, rather than cattle making us sick, we created a zoonotic bridge by animal domestication, "making rinderpest just as much an unintended side effect of human social development as measles" (p. 169). Similarly, human tuberculosis is surprisingly young, but bovine tuberculosis is younger still: we made our cows sick. And unlike a decade ago, we now have solid evidence that the bacterium Yersinia pestis did cause the bubonic plague of the Middle Ages.

Fourth, Harper brings similar nuance to conventional historical narratives. Agriculture has a bad reputation amongst historians. Palaeopathological findings, such as those gleaned from teeth, do not lie: our health suffered when we started farming. However, this narrative "distorts the role of infectious disease in the deep past" (p. 79). Agricultural villages were too small to sustain chains of transmission of respiratory crowd diseases: "Big city life, and big city death, were things of the future" (p. 162). What sedentary life with domesticated animals and without sewers did bring was mountains of poop. And with it the faecal-oral transmission of a slew of oft-overlooked intestinal diseases. Similarly, the discovery of the New World has been called the Columbian Exchange: we took away new crops and left behind lethal germs, an idea further popularised by Jared Diamond. Though true, focusing on disease makes "the depopulation of the New World seem like a lamentable accident, minimizing the role of violence and deliberate exploitation" (p. 243).

Fifth, this book highlights neglected (tropical) diseases and historical actors. The period from ~1670-1820 was an age of receding pandemics, often credited to the Enlightenment. But, adds Harper, the rise of institutions and nation-states was just as important. "Improvements in life expectancy are generated not by ideas alone but by ideas that are put into action, especially by capable governments" (p. 384).

Sixth, and related to this, history is rarely a linear march of progress and Harper highlights overlooked periods of setbacks. The age of receding pandemics was followed by new disease outbreaks in the 19th century – such as cholera, yellow fever, and influenza – all of which piggy-backed on the invention of steamships and railroads. Long-distance trade and bulk shipping of tropical foodstuffs created a new global disease ecology for humans, animals, and plants alike. "More often than not, the outbreaks of the nineteenth century came in peace" (p. 429). One of the most fascinating ideas in this book is that of 14th-century Arab historian Ibn Khaldun who recognized that epidemics do not just result from societies failing, but also from them being successful. Harper calls it a profound insight: at least in agrarian empires, demographic upswings also could trigger lethal epidemics and social disintegration.

Finally, Plagues Upon the Earth is rigorous yet readable. Harper draws on a huge body of academic literature, providing 90 pages of references and 59 pages of endnotes with interesting asides and advice on recommended reading. As above quotes show, Harper has a knack for writing an engaging narrative.

Though Harper grants it is too early to take stock of the COVID-19 pandemic, he reiterates that we were warned time and again. "the pandemic was a perfectly inevitable disaster [...] its contours predictable, its details essentially random" (p. 504). Our shared history with germs should be classroom material. Harper expresses this call to arms most powerfully right at the start: "We do not, and cannot, live in a state of permanent victory over our germs. Eternal vigilance is the price of liberation from infectious disease, but interruptions are inevitable, not anomalous" (p. 3).

Plagues Upon the Earth is a phenomenal achievement and my recommendation of the year for background reading on the COVID-19 pandemic. This is a big book that is well worth your time.