The Covid-19 pandemic caught most nations and their public health systems by surprise. Yet there has always been considerable historical evidence to understand the emergence of infectious diseases that should have left us more prepared than we were. 

That’s the subject of the recently released second edition of Emerging Infections: Three Epidemiological Transitions from Prehistory to the Present (Oxford University Press), which was co-authored by Ron Barrett, associate professor of anthropology. Among the first comprehensive syntheses of both the societal and environmental drivers of infectious diseases in humans, Emerging Infections employs a multidisciplinary approach to lay out this rich history. Professor Barrett shares what we know and how it can be applied in order to better prepare for the inevitable emergence of the next major transmissible disease.   


You point out that recent outbreaks of Ebola, drug resistant TB, and Covid-19 took us by surprise, but we should have been preparing for them a long time ago. What essential truths got missed? 

The essential truths have to do with recurring themes in the history of infectious diseases in human societies. The outbreaks you just mentioned are quite novel, but the determinants — the factors that brought them into being — are quite old and recurring. We should be aware of these recurring factors so we can better prepare for whatever the next pandemic is going to be. 


Disease outbreaks over the last 11,000 years have largely been determined by changes in patterns of human activity. The book breaks this activity down into three distinct periods known as epidemiological transitions. The first transition began 11,000 years ago in the late Paleolithic. What are its characteristics? 

The first epidemiological transition occurred with the intensification of agriculture as our primary form of subsistence. It wasn’t that human beings hadn’t farmed before, but when we began to permanently settle and intensively farm in single areas, create irrigation systems, and domesticate animals, it created a series of social lifestyle changes. 

Those changes included increased population density and closer proximity to animals over extended periods of time. Those animals carried novel infectious diseases. Many of the diseases we face today are descendants of these diseases. 

With regards to nutrition, the farming of cereal grains brought an important source of food energy for us, but it often came at the expense of dietary diversity. We see a marked increase in nutritional problems that are reflected in ancient skeletons. Unfortunately, we know all too well from living populations that any form of malnutrition is almost almost always accompanied by acute infectious diseases. 

For more than 100,000 years, Homo sapiens lived in small social groups. Large populations were not conducive when we were mobile and subsisting on hunting and gathering. You don’t get a lot of social hierarchy with small groups because if people don’t get along, they split apart. With larger-scale societies, we start seeing specialization of labor, social hierarchies and differential distribution of essential resources. These social changes brought differential health outcomes that contributed to the rise of acute infectious diseases. 


The second epidemiological transition coincided with the Industrial Revolution beginning in the late 19th century in Western Europe and North America. What changed in terms of the relationship between humans and diseases? 

Essentially, this second transition is a reversal in some of the changes that came with agriculture thousands of years earlier. New farming methods and transportation improvements brought greater varieties of food resources to more people, even those with lower incomes. This is not to say that poverty wasn’t a problem, it absolutely was, but there were significant dietary improvements overall. We see this as people of all classes began growing taller at much faster rates than could be explained by genetic factors. 

Improvements in water quality, wet and dry sanitation, and waste disposal had major impacts as well. Here, industrial societies were learning how to live healthier in large and densely populated urban areas. It didn’t happen right away, but in the late 19th century, we began seeing housing reforms that brought even greater infectious disease declines. 

Many of us believe that these health improvements were primarily due to biomedical discoveries, but this was not the case. Germ theory eventually led to effective antimicrobials and many of the vaccines we see today, but most of these discoveries did not happen until well into the 20th Century. Until then, the largest declines in infectious disease had more to do with non-pharmacological factors: changes in nutrition, changes in the way we were living together, and changes in the way that essential resources were distributed. 


Lastly, the third transition is the one we’re in today. It began not long ago in the last quarter of the 20th century. What important shifts happened here? 

Today, we see a significant rise in what are called syndemics, synergistic interactions between multiple diseases. Especially relevant are the interactions between chronic noninfectious conditions such as diabetes, heart and lung diseases, and acute infectious diseases such as TB, influenza, and of course, the highly pathogenic coronavirus infections. With regards to Covid-19, the highest mortality has occurred among older people with preexisting noninfectious conditions as well as depressed immune systems. 

Globalization is the other major shift. While the globalization of human populations has been occurring since about the 5th century, C.E., this process has greatly accelerated in the last 50 years with improvements in communications and transportation, as well as the growing interdependence of economic systems. We are now living in what is effectively a single-human disease ecology, which means that outbreaks in any society or any part of the world can quickly become a threat to everyone else. 


As you point out in the book, the primary lesson is that all of our infections, past and present, are essentially social diseases. If we acknowledge this fact, what do you recommend we change? 

One of the most important things is that we need systems for the active detection of infectious diseases, and small decentralized clinics that can provide basic public health services.

Disease surveillance comes in two flavors: active and passive detection. Passive detection is what most nations do – wait for somebody to walk into an E.R. or a doctor’s office before testing them. But the passive approach does not give us a representative picture of what is happening to the population as a whole. We can only know this with active detection, when health workers go out into the general population and randomly test for all manner of infections. We have the technology to do this now, and we can do so cheaply and at very large scales. Minimally invasive health surveillance is going to be essential for us to have a sense of what’s out there. 

Part and parcel to active detection is a well-functioning public health infrastructure. 

By this I am not calling for a particular type of healthcare system; societies can hammer out those kinds of issues. But people need not argue about having small, neighborhood clinics that are staffed by people who are known to their surrounding communities. These clinics can provide early information to health authorities about potential outbreaks. They can also serve as trusted sources of medical information for their patients. 

Trust is essential during an epidemic. I don’t think it’s realistic to expect people to trust national figures on public television, regardless of what conspiracy theories they may or may not have. But I do think they’re  more likely to trust their own local health providers.  

These kinds of lessons are very practical, and they should resonate with most people regardless of political background. Without this kind of common ground, we will not have the kinds of lasting health reforms that will be necessary to deal with future epidemics.

April 9 2024

Back to top