In the year 1900, life in the United States was very different.

One in every 125 people died of infectious diseases every year. It was a tragic fact of life that pneumonia and the flu regularly killed people.

But by 1980, that number was only about one in every three thousand. We take it for granted that over time, society gets better at tackling infectious diseases. But the fact that they became more than twenty times less deadly in just under a century is pretty astonishing.

With diseases robbing fewer people of long, healthy lives, life expectancy in the US jumped from 48 to 73 in the same timespan! Amazingly, we see a similar story in other countries too, which suggests something pretty big is going on. How did such dramatic changes happen in such a historically short span of time?

One way to approach the question is the theory of epidemiological transition. It describes how the nature of diseases in a country changes as the people and their societies change. In this episode, we’ll take a look at how this theory helps us chart the journeys nations take to push back infectious diseases, and the challenges they face along the way.

I’m Pardis Sabeti, and this is Crash Course Outbreak Science! [intro]. In the twentieth century, something remarkable was happening: populations were exploding. Not literally exploding.

The size of the population in many countries was growing rapidly. Population size depends on two key factors. There’s the rate at which people are born, the fertility rate, and the rate at which people die, the mortality rate.

If a population remains about the same size, those rates must be equal to each other so that roughly the same number of people are born and die every year. But for those countries which had growing populations, their fertility rate was larger than their mortality rate. And it wasn’t that people had suddenly started having more babies.

Instead, for these countries, their mortality rate had begun to shrink, causing the population to grow. And after some time, the fertility rate in those countries also tended to drop. People stopped having as many kids, and eventually the fertility rate was about equal to the mortality rate again, keeping the population steady at its new, larger size.

This process of falling mortality rates, later followed by falling fertility rates is what’s known as demographic transition. Some countries underwent this process in the twentieth century and earlier, while others are still experiencing it today. The big question is why.

As you might know from history class, in the last two centuries, most of the world has undergone some pretty dramatic social, environmental and economic changes. Technology has advanced, rates of literacy have increased, and the production of goods has grown a hundredfold, massively growing economies. And historically, demographic transition happens around the same time as these other changes in a country, suggesting that those factors are linked to the changing population.

On the fertility side, changes to the economic, cultural and social environments in different countries changed what kind of families people have and the number of children from generation to generation. People have lots of different reasons for deciding whether or not to have kids, but on average, they start to have less children during transition. But we’re going to focus on the other side of the equation, mortality rates, since that’s the part most affected by disease.

And it’s also where epidemiology can help us. Epidemiologists study how often diseases occur in different groups of people. We’ll take a closer look at their roles later but for now, the important thing is that since most people tend to die from some kind of disease, epidemiology helps explain why mortality rates fall during transitions.

Epidemiologists saw that before and after these demographic transitions, there were very different trends in the common causes of death. Initially, many people in a country seem to die from infectious diseases like tuberculosis or dysentery. But after a transition, the most frequent cause of death relates to diseases that aren’t infectious, like heart disease and cancer.

Those second kinds of diseases tend to be linked to the process of aging and our lifestyles. Nowadays, they’re often referred to as “non-communicable diseases” or “NCDs”, since they aren’t “communicated” between organisms by infectious pathogens. The shift from infectious, or “communicable” diseases to NCDs as a major cause of death is a vital clue for understanding why a country’s mortality rate could be decreasing.

It’s also where epidemiologist Abdel Omran comes in. In a paper published in 1971, he put forward a way of understanding this change with the theory of epidemiological transition. The idea was that the falling mortality rates during a demographic transition are due to a smaller fraction of people dying from infectious diseases relative to the whole population.

That’s why there are more deaths from NCDs proportionally, since people live longer and are more likely to die from an NCD instead. The theory also explicitly stated that the transition was linked with the broader changes that these countries went through, just like the theory of demographic transition. The kinds of jobs people have, their surrounding environment, the technology they have access to and the social changes that come along with them all impact people’s health and what kind of diseases they get.

The theory of epidemiological transition also recognised that no two countries transition in exactly the same way. In England and Sweden for example, the change was a gradual one over more than a hundred years, while Japan saw a similar decline in its mortality over a much shorter period of time. At the time, Omran proposed a few distinct models for transition to explain these differences.

Today, even though the evidence is broad and complex, we have a better idea of how the change in the sorts of diseases that influence mortality rates comes about. And we now see that epidemiological transition isn’t always straightforward, and there can be bumps along the road. We can see this by considering the transitions of two made-up countries,.

Johnovia and Hankistan. Although they’re made up, the scenarios they face are all ones that real countries have experienced. Let’s start with a little background.

Johnovia has a tropical climate, and people there live in big houses with their extended families. Hankistan, meanwhile, is more temperate, and lots of its population is concentrated in a few cities. Despite these differences, both of them are in a similar state.

Their populations, sadly, have a high mortality rate, and half of all the people end up dying from infectious diseases, hunger, or pregnancy-related health problems. Neither one of them is doing so great economically and people don’t live in the safest and cleanest of environments. But then things start to change.

Let’s go to the Thought Bubble. Both countries experience declines in their mortality rates, but for quite different reasons. Hankistan’s economy begins to industrialize and soon, on average, people are working more economically productive jobs and getting a little richer.

Hankistan invests this new money in better sewer systems and trash collection, taking waste away from where people live. Since waste was a major source of contact with infectious pathogens, this change ensures fewer people get infected. They also put money into healthcare systems that allow them to roll out vaccinations against diseases like smallpox and enforce sanitation measures in hospitals like handwashing.

The country secures better health treatments like antibiotics and rehydration salts. Now, even when people get infected with a disease, they’re more likely to survive. All of these factors make Hankistan’s mortality rate drop way down.

Meanwhile, in Johnovia the rats that used to carry fleas that transmitted plague are going extinct, so by pure luck, the country sees a decrease in its mortality rate. Separately, over time, Johnovia’s social landscape changes. People move from houses with their extended families to smaller individual homes, meaning that infectious diseases can’t spread to as many people in a household.

Hankistan’s investments in public health measures influence officials in Johnovia. At little cost, local and national governments start providing better guidance on quarantining and self-isolating to stop infections spreading. And when Johnovia’s economy finally begins to grow, too, they invest in public health, and adopt similar measures to Hankistan like vaccination and sanitation, but they only see a small decrease in mortality.

Like I mentioned, these transitions aren’t straightforward, and changes that helped one country don’t necessarily have the same effect on another. It isn’t until Johnovia begins producing and importing a greater variety of food that their mortality rates drop to the same level as Hankistan’s. With more readily available food, hunger, and the increased vulnerability to disease it creates, kills fewer people.

Thanks, Thought Bubble! All in all, things have come up for both Johnovia and Hankistan, but their luck could change. A deadly new virus could spread from animals to humans, leading to an outbreak in Hankistan that devastates parts of the population.

Their overall mortality would actually rise again. And meanwhile, maybe political tensions in Johnovia come to a head, ending in a civil war. The violence might shed blood and even destroy some of the existing infrastructure, undoing some of its benefits.

Mortality would rise there too. Eventually Hankistan and Johnovia could overcome these challenges. The war could end and preventative measures and treatments against the virus could take hold, improving both situations.

So far, we’ve looked at both Hankistan and Johnovia’s populations as though they’re just one big group. But in the same country, there can be populations who experience totally different situations. Since a lot of its population was there,.

Hankistan’s wealth became concentrated in its big cities, which experienced better health overall compared to poorer, more rural areas of Hankistan. A new virus would have a much greater impact outside of cities since health systems in those parts of the country wouldn’t cope as well because they lack the equipment and resources that bigger investment could have provided. Many real countries also have stark health inequalities within their population.

People live in different environments, are exposed to different risks and can be better or worse off when public health measures are implemented in response to an outbreak. And of course, it’s not like infectious diseases have disappeared for good after a transition. Johnovia, as we know, is a tropical country, and so has a naturally high number of mosquitoes, which is keeping cases of malaria high.

And what’s more, although fewer people are hungry thanks to their food policies, changes to lifestyles led to high rates of obesity, which could increase the likelihood of developing NCDs like heart disease and cancer. In fact, many lifestyle factors, like whether people smoke, drink, exercise, or sleep well, could affect how prevalent certain NCDs are in different demographics within Johnovia’s population. Countries that still face a sizable amount of infectious disease and NCDs are said to be facing a double burden of disease.

That’s particularly challenging because it requires tackling two related public health problems together. For example, having a chronic condition like lung disease or cancer can make you more susceptible to falling seriously ill or dying when catching an infectious disease like the flu. And even though countries might not have a high mortality rate from infectious diseases all the time, as populations get older, the prevalence of NCDs becomes high.

That means, in the case of an outbreak, a country with an older average population may be more vulnerable than it otherwise would be. So the same social, environmental and economic changes that drive largely-beneficial epidemiological transitions create new vulnerabilities. As we saw for Johnovia and Hankistan, the inequalities within and between countries can even increase the risk of outbreaks and impact who will be most affected by them.

And that’s what we’ll be looking at in our next episode. Thanks for watching this episode of Crash Course Outbreak Science, which was produced by Complexly in partnership with Operation Outbreak and the Sabeti Lab at the Broad Institute of MIT and Harvard, with generous support from the Gordon and Betty Moore Foundation. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.