How Diseases Shape the Course of Human History: A Conversation with Author Kyle Harper

Webinar: How Diseases Shape the Course of Human History: A Conversation with Author Kyle Harper
Air Date: February 15, 2022

Sabrina Sholts:
So my name is Sabrina Sholts. I'm a biologic anthropologist at the National Museum of Natural History and I am also a curator of the Outbreak exhibit that I just mentioned. I am a blonde-haired woman wearing a green plaid jacket over a black top and in my background is a bookcase to my left and part of a kitchen on my right. And on this screen is a photo of the cover of Kyle Harper's book Plagues Upon the Earth along with the date and title of today's event, How Disease Shapes the Course of Human History: A Conversation with Kyle Harper, and thank you so much for joining us.

All right. So as people start to trickle in a bit more, and it's great to see so many people joining, I'm going to go through our standard housekeeping notes for those who are maybe new to our programs. First, closed captions are available by clicking the arrow next to the CC button on the Zoom toolbar that should be located either at the top or the bottom of your screen. In just a minute, I'm going to introduce our speaker, Kyle Harper, who is going to give us a short presentation about his work and then we'll have a Q&A with the audience after that presentation. But please feel free to submit your questions at any time in the Q&A box on the Zoom toolbar. The Q&A goes by really quickly, so it's going to help us to answer as many questions as possible if you submit them as soon as you have them. And with that, we are going to get started.

Okay, so for five years now, the Outbreak exhibit and its associated programming have been educating the public about viruses that spill over to humans from other animals, and about pandemic threats for the last five years of newly emerging diseases, and how human, animal, and environmental health are all connected as one health. And I would like to take a moment to say thank you to the many organizations and individuals who have supported the outbreak exhibit among those years, including those that provided funding.

There are a number of epidemic and pandemic viruses that we feature in the outbreak exhibit, including now SARS-CoV-2. But there are many more pathogens that we don't discuss in detail, and that is simply because we did not have enough room in the gallery to discuss them all. In fact, there are very few, I would say epidemics from ancient history that we highlight, and given that historical perspectives are so important to help us better understand human pathogen interactions in the past and the present, we are very fortunate and very grateful to bring you programs like this one to have those conversations, and we could not be more blessed to have a more knowledgeable and distinguished expert on this topic than our guest today.

And so with that, I'm going to introduce Kyle Harper. Kyle is an economic and environmental historian who specializes in the ancient world. A professor, scholar, and Senior Advisor to the president and Provost Emeritus of the University of Oklahoma, Kyle's work tries to integrate the natural sciences, social sciences, and humanities to deepen our understanding of human expansion as a planetary force. And Kyle's also an author, and his fourth book, Plagues Upon the Earth: Disease and the Course of Human History, the theme of this event, the focus of our conversation today, is a global history of infectious disease spanning human origins to COVID-19. And you can purchase the book with a 10% discount from our local partner and book seller Politics and Prose using a link and code in the Q&A. And so with that, welcome Kyle. Thank you so much and please do take it away.

Kyle Harper:
Hi Sabrina, thank you so much for having me. Thank you for your amazing work in the outbreak exhibit and I feel like both the spirit of the exhibit and the book that I've written have a kind of common framework or at least a shared sensibility, so I'm really honored to be here and have the chance to talk about the history of disease. I am trained as a historian, trained in the history of the Roman Empire, but I got interested in the history of disease because the Roman Empire experiences a series of really major disease outbreaks and so I wrote a book that touched on that and it just got me hooked on the the importance of disease and the really intimate and profound role that they've played in human history as well as the questions of why do disease outbreaks happen, why do they happen when they happen, what are the ecological contexts that promote the emergence of diseases.

And so it was sort of those questions arising out of my work as an ancient history that I wanted to set out and write a big history of infectious disease and in doing so, one of the inspirations for me was a book that written in 1976 is still amazing, William McNeill's Plagues and Peoples. It's a really sweeping attempt to show the entire history of the relationship between humans and their infectious diseases, and it's an inspiration in the scope and scale and I think there are obviously things that you can see by zooming out, this is very much a kind of history that zooms out, and it's also a kind of history that's interdisciplinary. So that tries to combine insights from the natural sciences, from public health, from epidemiology, from microbiology, to help us understand human history more and it's a kind of history that is global, so it's broad in scope, both chronologically but also spatially and I think it's absolutely necessary if you're talking about the history of infectious diseases to try and have the broadest possible geographic scope and we'll come back to that theme.

This isn't a kind of history that's necessarily focused in on the details although as a professional historian I'm obligated to say that the details are often critical for understanding the big picture events and so whether it's understanding Roman papyri or ancient texts in Greek and Latin, the kind of narrow grained work on the historical sources for understanding health and disease remains just as important as we try and reconstruct these big stories but what I'm telling today is the very big story of human disease.

Now I think it's helpful to start with a few biological basics, what is infectious disease, it's a state of impaired health that's caused by something that invades the human body and there are biologically lots of different taxa or groups of organisms that are capable of invading the body and causing infectious disease. The most important are viruses that cause diseases like AIDS, yellow fever, coronavirus obviously, bacteria that cause plague, cholera, any number of diseases like tuberculosis and play an important role in infectious disease both past and present. Other organisms like protozoa, single-celled organisms that have an organized nucleus, maybe there are less of them but the burden of disease that they cause, for instance diseases like malaria, is simply enormous, and just as important throughout the human past, worm diseases, helminths that cause diseases like hookworm.

So infectious diseases are disease caused by an invader. The biggest evolutionary problem that a microbe has to solve is how to get from one host to another. They leave almost no road untaken, they are transmitted directly across skin. Some of the nastiest diseases are transmitted by disease vectors like mosquitoes but also fleas, lice, ticks and so on. They are transmitted in waste matter that makes its way into the digestive cavity of the next victim, usually but not always via contaminated water, so travel via the fecal-oral route. There are of course respiratory diseases that travel either via aerosols or fomites from one infected person to the next, and then a small but important group of diseases that have managed to figure out how to take advantage of sexual transmission.

Pathogens are microbes that are trying to replicate their genes, and they are using hosts to do so. Some of them are very narrowly adapted to a particular host, and some of them are more promiscuous and able to infect a wide range of animals. Pathogens are moving constantly throughout nature between wild animals, domesticated animals, and human hosts. When pathogens are adapted to us, we think of them as particularly human diseases, and when they're not adapted to us but they can cause infection in humans, they cause what we call zoonotic disease. And so Sabrina mentioned the One Health paradigm, which is fundamentally the idea that our health is interconnected with the environment and with the rest of animal health and disease.

So disease in human history, if we have to sum it up in a few really big themes, I would say first, people are strange from start to finish. That's one of the really I think most fundamental things we can take away from thinking about the big picture of human disease, and I'll say why we're strange in a second. We're strange in the biggest sense because of our history, because our history drives the evolution of pathogens and so one of the themes of the book and one of the themes I hope to touch on today is that infectious diseases affect human history but human history also affects the evolution of infectious diseases. So our history creates the ecological context that allows the emergence of infectious diseases, and globalization is one of the particularly important themes in the history of disease, because it's the story of how human societies become more interconnected and the way in which our health becomes more interconnected over time.

Infectious diseases are always interconnected with questions about power and wealth, and health inequality is a powerful force that is deeply implicated in structures of power and wealth both in the past and in the present day, and finally, evolution. The ultimate explanation for anything in biology is evolution, the ultimate explanation for the phenomenon of infectious disease is evolution, and evolution is pervasive, constant, and fast. It's simply relentless. I think that's one of the major themes of the big history of disease.

So let me start with the very basic fact that people are strange. It's always helpful to compare ourselves to other animals, particularly other primates, particularly other apes, particularly our very close relative the chimpanzee. And doing so can help us see ourselves more clearly and in fact if we compare ourselves to the rest of the animal kingdom, we're strange in the fact that we have a huge number of infectious diseases. By and large many of them are extraordinarily nasty and a really unsettling number of them are very narrowly focused on taking advantage of us. So I'd like to say our diseases are unusual in their number, nastiness, and narrowness.

Of course chimpanzees have infectious diseases. They have some pretty nasty ones too, and they have their own that are very focused non them, but humans have an extraordinary number of infectious diseases that are caused by viruses and bacteria that cause very severe disease so are highly virulent and for evolutionary reasons that we'll talk about, probably couldn't get away with being quite so nasty if we didn't live the way that we live in the kinds of habitats, in the kinds of urban settlements that we live in, and that have taken advantage of human success by narrowing their host range and so really can't cause disease or transmit reliably in other hosts but are sort of all in on an evolutionary strategy that's focused on the human host. So we have an extraordinary number of pathogens or disease causing microbes that are very, very narrowly focused on taking advantage of us, that learn the tricks of how to evade our immune system, and to take advantage of the human body as its reproductive vessel.

So why do we have so many diseases? Why are we strange, even compared to our closest relatives in the animal kingdom? Because of our history. Humans have history, meaning that we have cumulative, culture-driven, technological change over time. Chimps 100,000 years ago lived pretty much the way that chimps live today, whereas humans 100,000 years ago live almost inconceivably different lifestyles than most of us in the developed world live today and the basic reason is history and more specifically because of technological developments that have changed the basis of subsistence and the way that we live and move in the world.

And so our disease pool is inconceivably different than it was 100,000 years ago. Most of the major diseases that we think of when we think of the great roster of tuberculosis, plague, smallpox, cholera, AIDS, typhus, typhoid, salmonella, the biggies, didn't exist 100,000 years ago. They're products of the past, of the relatively recent past, and they're products of our evolutionary development over the last tens of thousands of years.

So our history, every time we've changed the way we extract energy from the environment, the way that we produce food, the way that we live together in villages or cities, the way that we dispose of waste, all of our cultural practices, affects the way that our diseases move in the world, so we shape ecologically the evolutionary conditions that give rise to infectious disease.

Let me give you two quick examples of what I call the infective paradox of progress because it's a simple principle of ecology that parasites are going to take advantage of hosts that are attractive, and human hosts are very attractive targets because there are so many of us, because we live so close together. So paradoxically, technologies that have allowed humans to multiply and to be more interconnected have also played into the hands of our parasites, and so when humans do things like shift from hunting-gathering to farming, they have to settle into sedentary societies, villages, then towns and cities, where they aren't moving around constantly and aren't moving away from their own waste products. This means that diseases that travel in waste of either humans or animals, particularly like the domesticated animals that we also at the same time we have been farming and settled down to live in sedentary societies, surround ourselves not only with our own waste but with the waste of giant domesticated mammals, attract an extraordinary number of fecal-oral diseases, and it's too easy to forget what an enormous burden of morbidity and mortality diarrheal diseases and dysentery have imposed throughout the human past and the basic reason is because of our lifestyle, because of the historical transition from mobile hunting and gathering to sedentary farming.

So humans have a really unusual number of diseases that travel from feces into the mouth of the next victim. We also have a really extraordinary number of respiratory diseases, and one thing we've learned in recent years is that many of our most famous respiratory diseases are in fact very young in evolutionary terms, products of only the last few hundred or few thousand years, and the reason why is because in order to survive, if you're a respiratory pathogen, whether a virus or a bacteria, you have the enormous problem of getting from the respiratory tract of one host to the respiratory tract of another. And when humans live in small scale societies and particularly mobile societies, that's a very enormous evolutionary challenge. It doesn't mean that there weren't any respiratory pathogens in the deep past, but as we have built cities and become more interconnected, it's made it easier and easier for respiratory to pathogens to take advantage of that and so COVID-19 is simply the last in a long line of novel respiratory pathogens that have taken advantage of human success in building things like cities and in interconnecting our societies.

And those interconnections are one of the great themes in the history of human disease. Globalization, which we sometimes think of in very narrow terms as a very modern phenomenon that was created by the internet or by very modern late corporate capitalism that helps to homogenize the world, is really a much deeper process of human connectivity, and so over the last 5,000 years or even more, human societies have developed technologies that have allowed them to trade and to exchange people and goods and ideas as well as germs, and so every step forward in human connectivity from the domestication of the horse to the rise of large scale empires to the globalization of trade in the Old World and then the explosive interconnection of the two hemispheres with the crossing of the Atlantic by Christopher Columbus that truly helped to globalize the disease pool of the world to modern globalization driven by fossil fuel transportation, the rise of steamships and railroads that freed human transportation and mobility from its reliance on horses and wind power, each step has helped to interconnect human societies. And when human societies are more interconnected, their health is more connected, and their disease pools are more interconnected.

So this is in a sense an inevitable downside of globalization which of course has enormous material and cultural benefits that more than counterbalance these. But every time human societies become more interconnected, it means that pathogens are able to exploit human populations and so our modern phase of connectivity that's driven so powerfully by jet travel that allows all of us to skip around the world more quickly than a pathogen can even incubate inside of us means that we are truly one global population whose health is highly interconnected. But this is again not something that's entirely new, and we can look to history for examples of how pathogens have taken advantage of human advances in transportation technology.

So again the 19th century is the age when humans learned to exploit the potential of fossil fuel energy to drive transportation machines like steamships and railroads and the invention of steamships and railroads connects global society in ways that are both good and bad and on the downside of course is that it makes it easier pathogens to transmit. So it's interesting that in the 19th century is actually when the word pandemic that we are so familiar with comes to have its modern meaning of a global disease event, and of course, that's because this is the time period in human history when the globe is becoming so interconnected that pathogens like cholera that becomes a global pandemic starting in the 1810s and then emanates outward in a series of wave over the course of the 19th century and is still endemic in parts of the world today, but also diseases like influenza that help human societies perceive the global interconnected and possibilities of global transmission of disease. So the 19th century gives us the steamship and railroad, and it's no accident that this period also gives us the word pandemic in its modern meaning.

Infectious diseases are connected to everything else about the way we live, including power and wealth, and I show you a chart that I think sums up almost more than any other chart possibly could about the changes that have occurred in the world over the last two centuries. Two centuries ago, in 1820, the entire world at the beginning of the industrial revolution was relatively poor and infectious diseases were the leading cause of death, and so in the early 19th century, infectious diseases killed most people most of the time in all parts of the world. Each one of those x's is a continent where we have at least the ability to make estimates for per capita incomes and life expectancies. You can see that human life expectancies are low because in a world where most people die of tuberculosis, malaria, typhus, typhoid, plague and so on, then life expectancies are low.

The great transition that's happened over the last two centuries is industrialization and the onset of modern economic growth that has raised per capita incomes. We see each dot is a country today, as well as more than doubled, and in many cases tripled, average life expectancies and so everywhere in the world today has experienced major improvements in life expectancy that are largely driven by a deep change in the causes of human death, and so set aside COVID-19, we live in a world where the leading causes of death are non-infectious causes. So cancer, organ diseases, other degenerative diseases and chronic conditions. This is a radical change from the way that people died and the way that people lived as recently as two centuries ago, and as you can see too, it's important to understand that this progress in controlling infectious disease is also bound up with economic development and so health and wealth are strongly correlated and this is a causal arrow that runs in both directions. Wealthy societies are able to have healthier populations, but healthier populations are also able to achieve economic growth, and we live in a world where health inequality around the globe is still a stark fact, which is both a problem of humanitarianism and justice as well as a threat in a world where our health is highly interconnected.

So even as human societies have learned to control infectious diseases, evolution never stops. How did humans learn to control infectious diseases so that a world in which most people most of the time died of infectious diseases became a world where most people instead died of chronic diseases, organ conditions, cancers and so on. The basic shortest answer if you had to answer in one word is science, the slightly longer answer is the science of infectious disease and microbiology that allows advances in public health, biomedicine, as well as general improvements in human well-being and economic growth, and so a number of interventions from the control of plague to the rise of vaccination that's first announced in a public forum in 1798 by the English physician Edward Jenner to the more general systematic understanding of germ theory, the recognition that infectious diseases were caused by invisible microbes that leads to revolutions in public health like the provision of clean water, one of the most important health resources of all, private changes in hygiene and the way people treat the body and the domestic environment, as well as improvements in biomedicine from antibiotics to oral re-hydration therapy that's so important in controlling sickness and death in cases of severe diarrheal disease.

So it's really been a combination of interventions from biomedicine and public health that have allowed humans to gain the upper hand. This also of course is one of the major contributing factors to the extreme rise in population growth, the fact that there were just over a billion of us 200 years ago and there are now nearly eight billion of us means that evolution has given pathogens strong incentive, stronger than ever, to adapt to human hosts and evolve the ability to take advantage of being able to transmit between human hosts and infect the human body, and we need look no further than simply the coronavirus family to see that diseases continue emerging and so there are seven now members of the coronavirus family that are able to infect humans, and you can see that in a sense, these have been so far as we can tell emerging more rapidly, even as humans have gained control over infectious disease.

So evolution never stops. I think one thing that that tells us is that the COVID-19 pandemic is not an anomaly. In some way, it's the strictest obedience to the principles of ecology and the laws of nature. There are so many humans so interconnected living so close together that there was bound to be a novel emerging infectious disease, and there's bound to be another, and if we learn anything from the COVID-19 pandemic, I hope it's that human health is globally interconnected and that our responses to infectious disease must always be technical and social. In other words, the extraordinary development of vaccines have helped start to bring some normalcy and have I think done an incalculably good job of helping to stop the pandemic but the pandemic of course has continued even after the introduction of safe and highly effective vaccines faster than anyone dreamed of, and I think that's reminded us what history could have helped us predict, that social and behavioral responses, the way that people understand disease, think about disease, change their own behaviors, and the amount of trust that they have in medicine and science are as fundamental as the technical and scientific solutions themselves. Thank you very much and I look forward to questions and conversation.

Sabrina Sholts:
Kyle, thank you so much for that presentation. That was fascinating.

Kyle Harper:
Thank you.

Sabrina Sholts:
And yeah, and so impressive. Again, folks, if you haven't read the book, please check it out and it's such a significant, substantial work and to be able to summarize the main points succinctly was really impressive. So thank you for that, Kyle.

Kyle Harper:
Thank you.

Sabrina Sholts:
So I have some questions, and folks, just as a reminder, please feel free to ask your questions in the Q&A box at the bottom of the screen at any time and then as those come in, we will get to them, but I will get us started. So personally, I really appreciated the emphasis you put on power and wealth as factors in infectious disease emergence and spread. I certainly find and always think that it's the people with the least power and the least wealth that seem to be most impacted and burdened by disease, and we see that through time. But thinking about those disparities and those inequities, and you as a historian, I'm wondering ... Considering the sources that you use, try to give me the big picture of what happened exactly, how many people died, who died, why and what else occurred?

Historical sources it seems can often present biased interpretations and accounts of disease events. When you're thinking about colonial administrative records for example or sensationalistic news media that are maybe scapegoating or blaming or in some way targeting one group as the source of the problem. How do you deal with those issues as a historian, when often the people who are suffering the most in an epidemic don't have the power or the platform to tell their stories?

Kyle Harper:
Wow, it's a great question and a huge one and I think ... There's a couple of ways I'd want to try and answer it. One way we can think about the importance of infectious disease, and I'll call to task my fellow tribe of historians. So instead of blaming others, let's just think about the way that ourselves, even historians, have treated the history of infectious disease. When you kind of read the canonical books on the history of infectious disease, think about what diseases are always featured. It's plague, smallpox, influenza, and while those are diseases that have affected a huge number of human populations globally, affected a huge number of people within society at all levels of society, that in itself is a kind of choice to focus on very specific diseases, and I think it's helpful to try and use other sources to think objectively about what diseases actually matter to people because it's not always the big sexy flashy glamorous diseases like plague which I get it, I've worked on plague, it's amazing, I still can't wrap my mind around how plague is able to do what it does, so I can understand that.

But what about a disease like malaria? Which on almost any reckoning must be one of the handful, I mean top two, three, four greatest causes of morbidity and mortality in human history. And yet it's an environmentally specific disease because it's the mosquitoes that are necessary to transmit it don't live everywhere, they're disproportionately tropical, and so it's kind of a classic example of a disease that doesn't always figure very prominently in the stories we tell and I think that's just the beginning. I mentioned that I have a particular interest in trying to highlight diseases of the fecal-oral transmission, and these actually did matter in huge parts of the globe. They're still important today, they're still one of the most important causes of childhood morbidity and mortality, particularly in developing societies and people don't even know the names of diseases like shigellosis, but again shigellosis should be ... Which is bacillary dysentery, it's a horrific bacterial disease that infects via contaminated generally water, sometimes food, and causes an extreme amount of suffering and is still very, very important outside developed societies.

So that's a long way of saying it depends a lot on what diseases you focus on. I think that's one really important filter, and if you're not talking about hookworm and yaws and yellow fever, these are all tropical diseases, diseases of poverty that in the past certainly included diseases like typhus, then you're not really talking about or even trying to talk about the human experience of disease.

So that's one thing I think that's really important is what disease is, but then also to try and use other sources, whether it's archeology, bones, DNA, texts that do try in some way to get you outside of that sometimes narrow top-down view.

Sabrina Sholts:
Thank you, yeah. That's a good point, and it leads to another question as you say, just maybe ... Don't focus so much on certain diseases, but how much caution do you need when you are reading historical questions or trying to determine what the cause of a disease was? When the people at that time maybe didn't even know? How would you even know what disease you're talking about?

Kyle Harper:
That's a huge problem for people who are working on the periods I work on, and we have a name for the problem, it's called retrospective diagnosis. How do you take a classic description of a disease, whether it's in a medical text where sometimes we have enough to go on, or more often the case, non-medical text. And so to take a very famous example, the plague of Athens that's described by the famous Greek historian Thucydides, in terms of ancient literature it's the most famous plague description. There have been something like 40 diseases suggested seriously as the cause. We have no idea what caused the plague of Athens, and it can be very, very frustrating. So there are some diseases that have such specific symptoms that you can sometimes sort of think. So bubonic plague is the best example because it has a fairly characteristic presentation that it causes buboes, these hard sacs of pus that form in the lymph node that's closest to the site of infection, but even with the case of bubonic plague, which is pretty particular, we really historians, really seriously genuinely weren't sure if things like the Black Death was caused by yersinia pestis bacterium that causes bubonic plague. So really good historians questioned this as recently as 10 or 15 years ago.

We're very lucky now that we have archeological DNA, so I won't go into it, it's interesting but basically science has advanced, the tools have advanced, to be able to very confidently extract the DNA of certain pathogens and this is a case where there's now essentially certainty of what disease this was but I'd say that's still the exception rather than the norm. A lot of the times we're kind of hypothesizing and working with very, very imperfect information because people in the past didn't understand that microbes cause disease, they didn't have the same lenses that they were looking through that we do, so it can actually be an enormous challenge even to know what diseases were around in the distant past.

Sabrina Sholts:
Yeah. And it is fascinating, it's amazing. As someone also involved in this work, what is possible now to recover from museum specimens, from archeological remains and other ways that we can actually fully sequence sometimes the genomes and pathogens to try and understand what the agents of diseases were. And you do a beautiful job of integrating all these different lines of evidence and sort of areas of scholarship when you're telling these stories. Are there any particular diseases or epidemics when you were writing this book that you enjoyed? Enjoyed in the sense, maybe either you learned a lot more in the way writing them or you maybe enjoyed that you were able to tell these stories in your book?

Kyle Harper:
Yeah. Well, you don't write a, whatever it is, very long book on infectious diseases without having a kind of fascination with biology, and so I'm lucky as a historian that I found a way to kind of bring an interest in biology, and you really, you have to be fascinated with the way that these pathogens work and how powerful and sneaky and clever evolution is. Now of course it's kind of unfortunate that the sort of scenarios we're looking at happen to be ones where pathogens learn how to fool the human body, but there's something absolutely fascinating about the ways that evolution is just constantly trying. I mean pathogens are constantly moving between different hosts, 99.9999% of the time they're failing, but occasionally they're able to adapt to the human host.

So I really liked thinking about and working with the measles virus. It's one where, actually a museum specimen, it's about 100 years old, just over 100 years old, was sequenced so this is cool because it's an RNA virus which is pretty hard to get, haven't been getting them archeologically but this was in a lung that was preserved in Berlin of somebody who died of measles 100 years ago. So it's an interesting RNA sequence, and it also lets us kind of date the measles virus and one thing that's cool that emerged out of this is that the measles virus is not that old, it is a morbillivirus. It probably adapted from an ancestor that could maybe infect cattle, more distantly rodents and bats. So we can kind of piece together the whole family tree of how this thing was adapting, wild animals, domestic animals, and then what's interesting is the latest estimate for when it adapted to transmit between humans aligns very closely with the kind of early Iron Age when humans start building really big cities.

And one of the things we already knew about measles, just from its biology, measles is insanely contagious. I mean the omicron is kind of in the same ballpark, but there's not many things that are like measles, and measles should not really exist. It is such an evolutionary freak. It transmits so quickly between anybody who's around somebody who's never had measles, they're susceptible. It's just sort of the most effective, it takes a very little dose to infect somebody, and it gives them very, very good resistance. So the thing about measles is if you get it and you recover, you're safe usually for the rest of your life, and it's one of the reasons why the measles vaccine is so effective and doesn't require a bunch of boosting.

So measles has a very bad evolutionary strategy, unless you're infecting humans who live in cities, and it's a cool example of how these new technologies are letting us see this kind of hidden history and the biology and the history can come together and they show us how humans create an ecology, in this case cities, where you have hundreds of thousands of people living together, and you have this pathogen that was out there, and its ancestors may have infected humans, but it really takes to us when we create the circumstances that let it take this otherwise kind of crazy evolutionary path.

Sabrina Sholts:
Oh yeah. No, that is fascinating. And we're getting a lot of questions here, so I'm going to start asking from the audience in a sec, but one follow-up there. Because brilliant study, and that just came out a few years ago, right?

Kyle Harper:
It was 2020, Dukes et. al. in Science.

Sabrina Sholts:
That's right, that's right. And curious, what were historians or scientists ... How old were they thinking the measles virus was before you're able to show exactly when it emerged?

Kyle Harper:
Good question, and to get a little technical, these estimates of when it emerged are really sensitive to the sequences that you sample. So before we had this older RNA sequence of the measles virus, more or less recent samples made it look like, the molecular clock estimates, made it look like measles had emerged more like 1,000 years ago. So this actually moved the timeline back a little bit and of course it can move again with the next study, that's one of the things in this field.

And so it does kind of change the way we might understand the historical context of its emergence, but it's a cool case where it has shifted even in the last few years, and I'm certain it will probably shift again.

Sabrina Sholts:
Yeah. Wow. Okay, we are going to start taking questions from the folks in the audience, and actually our first one I think nicely relates to actually what we're talking about, about humans living in cities. It's from Neils, and Neils asks, "Do any animals that have followed us into cities, like rats, pigeons, mice, have a similar profile of infectious disease as we do?"

Kyle Harper:
That's a very interesting question and you're right to bring up rats. Rodents are kind of the shadow of human progress, and particularly a handful of rodents that spread worldwide that are kind of hitchhikers on our success, like the black rat. Rodents are a really important source of infectious diseases, ultimately as a reservoir. This is actually true of bats and rodents, so we sometimes don't realize this, but most mammals are bats or rodents. There's huge numbers of species, and then there's tons of them, and they live in groups. So they're a really important natural reservoir where all sorts of terrible evolutionary experiments are happening all the time and we kind of bring them into our midst and so give them the opportunity to get our diseases and to give us diseases.

Now rodents don't have the same burden of infectious disease or the same kind of disease profile exactly that humans do. There's various reasons for that, for one thing most of them have really short life histories, so they're just being born, reproducing and dying really, really quickly, and so they don't have the same space to get some of the infectious diseases that we get. But there are important connections between human health and rodents and the most important one is a disease that we have alluded to, is bubonic plague. I said it's a really weird disease that I can't wrap my head around because it never adapts to humans and so it can transmit between humans but not stably, there's no human population that's a reservoir for yersinia pestis, it's a rodent disease and it's always a rodent disease. It's a wild rodent disease that domestic or commensal rodents like black rats are susceptible to, and these huge plague outbreaks in the past like the Black Death are really rodent disease outbreaks that humans get caught up in when the rodents die and their fleas are desperately hungry and jump to humans and bite humans. So it's an interesting question, there's some really important connections between human health and rodent health.

Sabrina Sholts:
Okay. Well, our next question is really getting into ... going from sort of how we've been shaped by diseases and sort of how history plays out as a result of them, right? Anne wants to know, she heard an argument a while ago that the 1918 flu pandemic was a catalyst for the U.S. government granting women the right to vote because in short so many men had died and women were in new positions of work in power. Do you know if that's true? Do you have any thoughts about that?

Kyle Harper:
I'm not an expert on that particular question but I can offer a kind of general reflection, which is as a historian, we always want to try to think in a careful way about causation, and infectious diseases can have really important consequences, they can be a driving cause in really important social changes. But there are two really different possibilities of the form that can take. One, diseases can really change the direction that's something happening and two, diseases can really accelerate or intensify. And so without being an expert in that particular question, I'd feel confident in saying that this is most likely to be a case of the second question, where the experience of an infectious disease outbreak, a pandemic that causes a lot of sickness and death, that's very psychologically traumatic for societies, that dislocates workers, kind of intensifies things that are already happening. So women's suffrage doesn't sort of..it's not a change in direction, it may have kind of accelerated or intensified things that were already there.

I think when people are looking back at the COVID-19 pandemic, they're not going to say that that COVID-19 caused the Black Lives Matter movement and the racial reconciliation movement in 2020, but clearly there was some sense in which the intensity of the disease experience sort of intensified the energy for reform, at least in the United States. And so there is some kind of connection, but it's not like the disease outbreak caused that. They happened at the same time and there's some sense in which that may accelerate history or intensify the direction things are going. I suspect that's what happened in this case that you're talking about.

Sabrina Sholts:
Really good point, yeah. It's difficult to disentangle different factors, right? 1918, we also had World War I for example and I imagine it's challenging to say one thing over the other. But that was a good question Anne, thank you. Okay, so now we have a question from Margaret, and she wants to know about the very recent emergence of multiple nasty coronaviruses, that graph that you showed, right? With all the different ones. What exactly did we do in the last two centuries to set off this rapid evolution and growth of coronaviruses in particular? We talked about globalization, urbanization. What would you say?

Kyle Harper:
Yeah. That's such a great question. And I would start by saying that a chart like that that's showing the seven coronaviruses, those are the seven coronaviruses that we know of that are capable of infecting humans. There must have been throughout history other coronaviruses that adapted to human populations and then went extinct, and so we have to remember when we're looking at something like that, there's parts of the past that we're not seeing. So it's not like coronavirus has probably just come out of nowhere over the last few hundred years. But what happens is as there are more and more people and we are more and more interconnected, there's more chance for them to adapt to humans, so the sheer number of times when a human encounters a bat or some other intermediate mammal that could have been a conduit for the coronavirus, if there's eight billion people, there's huge greater opportunity for that evolution to happen because it can contact people and then if people are living in bigger cities and they're more interconnected, it has more bodies to sustainably transmit within, more chances to evolve and this is one thing, we've all now got this very realtime lesson in the speed and significance of viral evolution in living through COVID.

We now all know what a variant is, and that's evolution at work, and something like the omicron is a very fit variant, so it's very good in evolutionary terms at doing what it does, at spreading its gene, and the reason why we have these very, very fit variants is because there are billions of people infected and the virus has a lot of chances to shuffle the cards of its genome and see which combinations work. And 999,999,000 out of a billion do nothing or make it worse. But every now and then, there's some little genetic change that makes it better, and so it's simply a question of the number of times it has to play the game. So as there are more people, they have more chance to practice these evolutionary experiments.

So coronaviruses look like they've emerged in realtime. Those are just the winners that have sustained, have been able to transmit stably in human populations, and it's because of us, because of the way we live. And I'll just share this one little story too. And you mentioned you've had the Outbreak exhibit running for five years. That means that it was running -

Sabrina Sholts:
Yeah. Programs for 4... yeah, we opened in 2018.

Kyle Harper:
2018.

Sabrina Sholts:
Yeah.

Kyle Harper:
So if everybody had come to Washington and seen it, maybe we would have been more aware of the inevitability of this, and I remember a conversation I was having in the spring, it was May of 2019 with a virologist, and I was talking about the potential for different pathogens to cause a destabilizing pandemic and was talking about influenza which I think does remain one of the threats to do something terrible. And he said to me, "Yeah, but look out for the coronaviruses. There's something about them, obviously they evolve very quickly, they move easily between animal hosts, and just a disconcerting number of them seem able to adapt the ability to attach to human cells, and so there's just something about coronaviruses." So the coronaviruses do seem like they have certain just kind of basic properties that make them able to move around between animal hosts really easily.

Sabrina Sholts:
Well, we do have a related question actually. Sort of again concerning what we do and sort of how we've impacted our ecology and changed our environment. Looking at the history of pathogens, can we make any predictions about the impact of climate change on pathogens that affect humans?

Kyle Harper:
I mean this is a trillion dollar question and we know that the climate system and health, human health and animal health, are very, very interconnected. We know, my perception is that we know, less about the answer to that question than you would think and we know less than we should. But it's a major research question, and so I'm optimistic that over the next five or ten years with the amount of human intelligence that is going to precisely that question, that somebody's going to be able to answer that question well over the next five years or so or at least to have a more structured answer, and there are people obviously who have been working on this, Anthony McMichael has been thinking about this for a long time. So we have some sense of some of the mechanisms but the answer is very, very complicated.

I'll say one way in which I think probably is the most kind of tractable right now is that a number of infectious diseases are very geographically sensitive because their vectors, usually insect vectors, that transmit them have very specific environmental requirements, and so this is why so many diseases are tropical diseases is because the warm and wet climate of the tropics is conducive to mosquito abundance.

So we know that as the climate changes, that's going to change the latitude and the altitude at which mosquitoes survive under abundant, and that that's going to affect human health. So we know that climate change is going to expand the territory of many important disease vectors, and that that's going to have consequences. Probably though, let me say one more thing. This is a huge and big question. Probably the most important mechanisms are going to be directly on climate to human well-being, which is then going to reverberate. So if the climate crisis causes poverty, subsistence crisis, migration, conflict, that's how it's most likely to affect human health.

Sabrina Sholts:
Yeah. Yeah. Thank you. That is a big question. That was a question from Vicky actually, so thank you Vicky for that, yeah. Huge, huge area of research. Okay, we are getting close to our time unfortunately but we can ask at least one more, and I've got a good question here from Eliah, who wants to know what can we do to avoid new viruses that can be so pathogenic? Don't we need to know for example what the viruses are that are so prominent in bats and other vectors?

Kyle Harper:
Yeah. I mean I think it's frustrating that the response to COVID-19 from say the federal government so far seems that the lessons we're learning to prepare for future pandemic preparedness seem to me quite narrow and so the Biden administration proposed a future pandemic preparedness plan that had a lot in it to like. I recommend looking at it. There's ways in which it's very attractive and ambitious, it's very biomedical focused so 95% of it is very much how can we develop testing, vaccines and therapeutics really quickly. That's hugely important and that's the heavy weaponry. Our world would still be much more chaotic than it is if we hadn't had the extraordinary biomedical response to COVID-19. So as a historian, I think we'll look back and say it was amazing, the scientific response.

But where we failed is really the social side, and both in understanding and preventing the emergence of disease and then responding collectively to the challenges of a pandemic, and so future pandemic preparedness probably always has to focus more on surveillance and exactly as he was asking about, understanding what are the sources of zoonotic threat. There's been a huge amount of work on that, but probably could be orders of magnitude bigger, and then surveillance systems to understand how microbes are moving earlier, right through to the kind of social behavioral interventions that might be needed to minimize the risk of exposure and emergence to the social behavioral responses that are going to be necessary at different stages of the pandemic response. So that's what I was getting at in my conclusion, that the technical side of our response to COVID-19, particularly in the development of vaccines, but also as we'll I think increasingly see in rapid testing and viral therapeutics, is going to be historians looking back on it in a century will give it like an A. It really has been amazing, but the kind of social and political responses are more like an F and it just seems like when we look to the next pandemic, there needs to be a lot more attention on addressing those from prevention to response.

Sabrina Sholts:
Thank you, and I think we could fit in one more -

Kyle Harper:
All right. I'll talk quick.

Sabrina Sholts:
Because we have a budding historian who's asked a question.

Kyle Harper:
Oh, good.

Sabrina Sholts:
So we got to take it. This is from Allegra, and so Allegra is wondering, she says thank you by the way, Kyle, about gaps that you found in your research. You're discussing specific diseases now and maybe that's one answer, but she's wondering what questions came out of this for you? What's next for you, Kyle?

Kyle Harper:
Yeah. Well I can answer that easier. Something about the history of animals and biodiversity and definitely that's coming out of this work and really as a historian being struck by how important animals and animal health is to the environment and the way human societies live. So I've gotten very interested in animals, and I actually think that's a way to answer her first question, that we've still really, despite the one health paradigm and the advances that it's helped in making us think about how interrelated human and animal health is, I still think we, including me very much in my book, tend to think of diseases too much in terms of humans and microbes and it really is going to be ... So future historians I think will hopefully do a better job of doing even more to bring wild animals, domesticated animals, all the commensal animals like rats and pigeons and stuff that we said live around us, and understanding how our health is really sensitive to the environment and the animals that live around us.

Sabrina Sholts:
Yeah. I agree. One Health, right? Thank you so much Kyle.

Kyle Harper:
One Health. Thank you Sabrina.

Sabrina Sholts:
Yeah. That is all the time we have, and if you want to say one last word, I can give you that.

Kyle Harper:
Thank you so much. I appreciate you and the amazing museum and the great exhibit. I hope everyone gets to it. And thanks to the listeners very much for their attention and great questions.

Sabrina Sholts:
Oh, thank you. And I apologize to those who asked questions and we didn't have time to address them. That is the tragedy of the one hour program, but please join me in thanking Kyle please in the Q&A if you'd like and I would also like to give a special thanks to those who made today's program possible, our donors, volunteers and viewers like you and all our partners who help us reach and educate and empower millions of people around the world today and every day. So thank you everyone.

Our next upcoming outbreak webinar will feature HIV/AIDS grassroots leader Peter Staley, writer and filmmaker David France, executive director of the World AIDS Museum Requel Lopes, and journalist Linda Villarosa on Wednesday, March 9 at noon Eastern Time, so there is going to be a link for that in the Q&A and you will also see a link to a survey and we hope that you will take a moment to respond. We're really interested in your input. It's going to help us deliver more great programs like this one in the future. So thanks again and hope to see you all next time. Have a great evening.