Dr Kyle Harper is a historian of the ancient world whose work has spanned economic, environmental and social history. He holds the post of Professor of Classics and Letters and Provost Emeritus at The University of Oklahoma, and has published four books on the history of slavery, sexual morality, climate, disease and empire. His latest, "Plagues Upon the Earth: Disease and the Course of Human History", is published in October 2021 by Princeton University Press.
You write that humankind’s ability to control infectious diseases is a relatively new thing. What intensity of fears might have stalked those who came before us? Was every handshake or doorknob potentially lethal?
It’s certainly new, in the grand scheme of human history. It’s easy for us to forget that down to around 1900, even in the most economically developed societies, infectious diseases were collectively the leading cause of death. Throughout our past, diseases like malaria and tuberculosis, typhus and typhoid, smallpox and measles and plague, and a lot of other diseases caused by microbial pathogens were the biggest burden on human health. Of course people feared sickness and were surrounded by threats in a way that is I think hard for us to understand. But recall that they also did not have our concept of germs – of microscopic agents of infection. They thought of disease as a kind of imbalance in the body, perhaps caused by poor diet or bad air or a weak constitution. I don’t think we should imagine most people in the past as “germophobes” in the contemporary sense!
You make the point that our mastery of disease and other illnesses is geographically uneven. Viruses know no borders, but quality of treatment, in a sense, does. Has this worsened or improved?
It is important to think about health inequality historically and to consider both within society differences and between societies differences. Between-society inequalities in health are a product of both geographic and human factors, and the interplay of these over very long time-periods. In the modern period, say from 1500 onward, the gap between the most and least healthy countries grows. In the 19th century, the gaps really widen, both because of rapid improvements in countries that industrialized first, but also because the disease environments in much of the world became much more challenging then too. The increase in global trade and in imperialism may actually have lowered life expectancies in many places. I argue that the global gaps in life expectancy may have peaked around 1900. Since then, there has actually been convergence, thanks to the global spread of science and biomedicine and public health, decolonization, and economic development. But, obviously, there are still large inequities today, driven in part by the burden of infectious disease. It is too easy to take for granted basic things like clean drinking water (one of the most important foundations of good health) that remains unevenly distributed.
Several factors have spurred an evolution in diseases that affect humans, including population growth and the development of agriculture. The latter allowed humans to become more sedentary. What happened then?
It’s actually an old trope that the transition from hunting and gathering to farming was a calamity for human health. In the 1970s Marshall Sahlins started calling hunter-gatherer societies the original affluent societies, and in the 1980s Jared Diamond called farming humanity’s greatest mistake. It’s true that farming was bad for human health in lots of ways, but we are starting to learn a lot about how changes in human societies shaped the ecology and evolution of our pathogens. I would argue that we have underestimated some of the effects of sedentism on disease environments. Among the many ecological implications of settling down from a mobile lifestyle to a settled one is that our ancestors started living in close proximity to their waste, and the waste of their domesticated animals. There’s really nothing in nature quite like the aggregation of feces in human settlements. And the evolutionary reaction to this is that we get a lot – a lot – of pathogens that exploit the fecal-oral route of transmission. The burden of diarrheal disease and dysentery throughout human history is enormous, and it’s a consequence of the environments that we create.
A later evolution changed human ecology once again: the rise of early cities. What did the Alexandrias and Constantinoples of the world do to our vulnerability to diseases?
The rise of big cities – which really takes off to new levels in the Iron Age – was an important transition in the history of disease. To a microparasite, the human body is a host. The more of them there are, and the more densely packed they are, the easier it is to exploit them. So the rise of dense aggregations of human bodies is the ecological background to the rise of a number of directly-transmissible pathogens – ones that transmit from host to host, often via the respiratory route. Measles is a perfect example. It becomes a human pathogen just as humans figure out how to build big cities. The measles virus is a freak of nature. It is completely specialized in infecting humans. It confers very strong and enduring immune resistance in survivors. It is extremely contagious and moves through susceptible populations quickly. In short, it has an evolutionary strategy that only works because of us – because we built big cities. Measles goes rapidly extinct whenever it is introduced into small, susceptible populations. Measles exists because of human social development. And now we have pretty remarkable consilient evidence for this from different fields. A new genetic study suggests that the measles virus adapted to human populations around 2500 years ago. That’s just the moment when human societies start building cities big enough to keep measles circulating for a long time.
Population change as a result of mass death from plague is one obvious example of how disease has shaped human social evolution. What are some of the less obvious examples?
The relationship goes both ways. Structures of power shape human health, while infectious diseases also shape power dynamics. Again, this is true both within and between societies. The whole history of colonialism, slavery, and imperialism was shaped by these two-way relationships, fundamentally. And disease has also had a strong impact on things like fertility and human capital. For instance, in really high-mortality environments, women have to marry earlier and have more children. There is a very strong association and I think causal relationship. And we now have a fairly firm understanding that the association between health and wealth goes both ways: yes, economic growth enables better health, but good health is also very conducive to economic productivity.
Technology has advanced humankind in myriad ways. But it’s also, in many ways, been catastrophic for our health. Tell us how.
It’s the paradox of progress. We can see the connections between the history of technology and the history of disease by trying to see both from an ecological perspective. The technologies that make human societies more populous and more prosperous also make us easier and more appealing targets. Think of a technology like the railroad. The use of mineral energy to fuel transportation was revolutionary. It was also a boon for our germs. The railroad - and the steamship - facilitated the transmission of infectious disease in the 19th century. The evident example is cholera, of course, which dispersed globally over rail and by steamboat. But influenza dispersed further and faster in the age of rail. And even diseases like malaria seem to have taken advantage of thicker networks of communication. Toward the end of the 19th century, by then in the steamboat age, bubonic plague made a bid to become a major global scourge again. Aggressive preventative measures kept it from flaring into disaster in most of the developed world, but the impact in east and south Asia was severe. In our own time, jet travel has similarly changed the connectivity of human populations and thus our pathogens. The dialectic of technological progress, pathogen evolution, and human adaptation is a fundamental theme of human history.
As we push technological innovations further, will we become more susceptible to disease?
Any technology that exposes us to more potential new diseases - by, for instance, intruding on new ecosystems - or that facilitates easier transmission of diseases or makes us more numerous, will create the evolutionary incentives for diseases to take advantage of us. Of course there are no guarantees – evolution is weird and unpredictable. And, of course, our tools continue to advance too. I think when we look back on the Covid-19 pandemic we will realize that it helped to stimulate a lot of amazing research and development. Hopefully the discoveries and technologies that we are making now will help us be more resilient in the future.
You’ve spent much of your career as a historian looking backwards, so to speak. But this issue compels us to look forward. How do you see our interaction with disease evolving over the coming century?
Of course I want to plead that I am a historian, not a futurist. But the past can help us see longer trends. We should have been more prepared for Covid-19. We were repeatedly and continuously warned by experts in microbial evolution, zoonotic disease, public health, and so on. And we were warned by historians, too. One of my favourite books is William McNeill’s Plagues and Peoples, which ends on a note of warning that infectious diseases have always been, and will always be, a factor for human societies. The reason why is evolution. Evolution is relentless, crafty, and weird. If there are more and more humans, microbes have the means, motives, and opportunity to adapt to exploit us. The modern period is not one of linear triumph for humanity. Thanks to empirical science and rational statecraft, we have gained a huge amount of control over infectious disease. But nature does not stand still. I think the big question is whether COVID-19 is the arrival of a new phase where we struggle to respond to microbial threats. And I would add that social adaptations are as important as technical solutions. Three extremely safe and effective vaccines were developed against Covid-19 in less than a year. The scientific response to the crisis has been tremendously strong. The social responses have been all over the place, with some communities and societies responding very well, others very poorly. We know new infectious diseases will arrive. We know our technical solutions will continue to advance. I worry more about our political capacities in the next few decades.
