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We’re all susceptible to infectious disease of some kind or other, but not everyone is equally likely to be the victim of an outbreak. The fact is, inequalities both between and within communities mean that some people are at higher risk than others. In this episode of Crash Course Outbreak Science, we’ll look deeper at the factors that lead to this uneven distribution of outbreaks.


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Everyone is susceptible to  infectious diseases of some kind.

But not everyone is equally likely  to be the victim of an outbreak. It’s vital to know where outbreaks  of infectious diseases tend to happen and who is most at risk of  being infected and falling ill, so we can target our strategies to prevent  outbreaks and help those most affected by them.

Sometimes the answers to these questions lies  in biological differences between people, like differences in immune system function. But often, the patterns of who gets infected  during an outbreak are linked to the social, economic, and environmental  differences between various groups. In the last episode, we saw how the course  of development across different countries helped decrease the fraction of people  who die from infectious diseases.

But those changes haven’t affected  everyone equally and in some cases, have actually increased the risk of outbreaks. While development has brought some   overall benefits like better  treatments and infrastructure, the access to those benefits between privileged  and marginalized groups has been unequal. In fact, inequalities due to factors like  where people live, what jobs they have and even their gender have meant that  some outbreaks devastate some countries while barely affecting others, or that individual populations within  a country have become a lot safer while others have been exposed  to new risks of outbreaks.

Today, we'll take a closer look into the factors  behind this unequal distribution of outbreaks. I’m Pardis Sabeti, and this is  Crash Course Outbreak Science! [Theme Music]. Since 1950, the global population  has grown from over 2 billion people to just under 8 billion people.

That growth exists in a kind of feedback loop with  our lifestyles, opportunities and surroundings. Changes in those things affect population growth but are also changed when the population grows. So there are important social,  economic and environmental changes that come along with the process of modernization.

And each of these factors, or determinants,  influences how likely outbreaks are to happen and who they happen to. Let’s start with the social factors, which are characteristics about  peoples’ lifestyles and behaviors, like where they live and work and  how they interact with one another. For starters, when there are more people, there are more interactions between them  and their surroundings, including animals.

That can spell trouble because 60% of the  emerging infectious diseases that infect humans come from our interactions with animals. This process of diseases jumping from animals  to humans is called zoonotic spillover. So if there are more people living in  close contact with livestock and wildlife, then they’re at greater risk of being the early  victims of an outbreak from spillover events.

Then there’s one of the biggest  trends today: living in cities. After all the population growth  we’ve seen across the globe, about half of the world’s  population now lives in urban areas. In fact, the UN estimates that  number could be nearly 70% by 2050!

At their best, cities can be places where  people can access better healthcare, opportunities and services, making  them resilient to outbreaks. But when lots of people move into a city over a  short period of time, it can also cause problems. A sudden population surge in cities might  mean that people need housing, fast.

If there aren’t enough houses, many people could end up living in the  same home in overcrowded conditions. Then, if someone contracts a disease, it can  spread more easily from person to person. And if cities can’t construct housing fast enough people might move into informally built  structures that don’t provide decent shelter or properly protect people from  vectors of disease like mosquitoes.

Meanwhile, local governments might not  be able to provide the services needed to tackle the increase in waste being produced or create infrastructure to support basic  sanitation measures like clean water access. All of this creates a hotbed for  infectious diseases to flourish. So we can see how our access to resources  plays a big role in who’s most at risk.

We’ll talk about this more in a bit. But even when resources aren’t the issue, social behaviors also shape the  likelihood of disease outbreaks. For example, in the twenty-first century, international travel is a way bigger  deal than it was a hundred years ago.

Lots more people go jet setting to places  they’ve never been to see the rest of the world and to work. Unfortunately, as many of us have experienced  first-hand during the Covid-19 pandemic, more people traveling also means that diseases  are more likely to spread internationally too. We saw the same thing with SARS in 2003,  another kind of coronavirus pandemic.

In Vietnam, Singapore and Canada, outbreaks of SARS were linked to  travellers flying back from abroad. Then there are the social norms  that influence our everyday behavior and our chances of getting infected. For example, gender roles have often shaped  how outbreaks of Ebola hemorrhagic fever first emerge and then spread.

In some countries, work undertaken outside of  the house involving close contact with nature   is more often done by men, while caregiving is more often performed by women. So outbreaks in those regions  often affect men first since they’re exposed to the animals  from which zoonotic spillover happens. But once the outbreak is underway,  women tend to be at greater risk, as they become more exposed to infected  people in their caregiving roles.

Gender, along with things  like race, ethnicity, and age are also the kinds of sociodemographic  differences epidemiologists consider when determining the cause of an  outbreak and how a disease is spreading, which we’ll take a closer  look at in future episodes. Finally, there are more subtle but  equally powerful social factors   that can influence the likelihood of an outbreak: our beliefs and emotions. One sad but avoidable example is  that in countries all over the world, shame and stigma have worsened  the effects of HIV epidemics.

Two highly-publicized transmission routes  for the virus, are intravenous drug use, or injecting drugs into the body,  and through anal or vaginal sex. In some societies and in particular circumstances,   these activities have been  or still are stigmatized, meaning particular members of society are socially  disapproving of anyone involved with them. That disapproval results in an unwillingness  to help those who might contract HIV, either with the illness or  their general wellbeing,   even when it would stop the spread of disease.

For example, the US has held  back on harm reduction practices   that would make intravenous drug use safer, in part because of stigma  around recreational drugs. The fear of that disapproval and lack of  support means that those who do have HIV, or suspect they might, avoid  identifying themselves, which makes it harder for them  to access treatment or help. Some people forgo sexually  transmitted infection testing because confirming that they have HIV  might feel like an enormous burden, which in turn increases the likelihood  of further transmission of the virus.

In this case the social stigma attached  to certain kinds of behaviours, and, by extension, the virus itself, both  prevents some people from seeking out, and some governments and groups from providing, every possible option for  prevention and treatment. All in all, it’s clear that  the social landscape we inhabit isn’t just a backdrop against  which epidemics take place. They shape how disease outbreaks  happen and who is more or less at risk.

As I mentioned, we know that some  of these factors are a consequence of those who have resources and those who don’t. That brings us onto the economic  factors that affect outbreaks. One of the most important economic  trends in history has been globalization, the increasing trade between different countries.

That trade involves things that we buy and  sell, like material goods and services, but also things like ideas, cultures,  people and, of course, outbreaks. Consider shipping. About 80% of the goods traded between  countries are carried on ships, which use ballasts that fill  up and discharge with water.

It basically helps the boat  float more evenly at all times, making loading at docks and sailing easier. But sucking up that water in  one place and discharging it in   another is a huge hazard for spreading pathogens, like the bacteria that causes cholera. In fact, ballast water has been linked to  several outbreaks of cholera across the world!

The sorts of things we trade matters too. As we saw earlier, over 60% of emerging infectious  diseases come from human contact with animals. So it’s no surprise that the booming trade  in wildlife and livestock carries huge risks.

It’s suspected that demand for animals, which  drives people into higher contact with them, could be the reason for the zoonotic spillover  that leads to outbreaks of both ebola and SARS. While we saw in certain countries that  men are more likely to work outdoors, the reason to take up jobs working with  animals is often driven by economics. It all links together!

It’s worth remembering that  people produce and trade goods to secure better prospects  and resources for themselves. But not everyone has prospered  from globalized trade equally. Although on average, people across  the globe have gotten wealthier, huge inequalities exist between  countries and within them.

Wealthier countries and even  wealthy regions within countries   tend to have better access  to sanitation infrastructure which prevents people from  getting infected by pathogens. But they also have better equipped healthcare  facilities and physical infrastructure like roads that lets people get to  hospitals easily if they do get sick. Economic factors also apply to individuals.

For example, access to jobs can affect  not only how much money you can make, but also how well you can cope with an outbreak. In the US, for example, there’s evidence  that social distancing guidelines didn’t work as well in low-income  neighborhoods during the Covid-19 pandemic. In those neighborhoods, people were more  likely to work in jobs that required them to leave their houses and interact with others, like childcare, housekeeping,  and restaurant service,   increasing their likelihood of catching the virus.

People in high-income neighborhoods, meanwhile, were more likely to have jobs that allowed them to   work from home or the savings  to take time off from work. Even if everyone wanted to follow  the guidelines just the same, for people with less wealth  it was simply much harder! Wealthy countries, and the  relatively wealthier people in them, also tend to be less susceptible to  our final outbreak-fuelling factor: environmental conditions.

A lot of the energy and raw materials we use  to make and do things in modern economies comes from extracting huge quantities of  natural resources from the environment. That process of extraction often comes at a cost. One case is deforestation, which involves  cutting down woodlands for the land or wood, destroying the natural habitats  of animals living there.

That often drives them out in search of  different places to live and find food, which ends up being close to where humans live. As you might suspect, it increases the  chances that diseases spread from them to us. For example, rodents driven out from natural  woodlands near deforestation sites in Brazil have been linked to the outbreaks in the nearby  populations of the deadly disease Hantavirus, which infects the lungs.

So, places that rely on extracting natural   resources in ways like this  to support their economies tend to be at greater risk  from this kind of outbreak. And what’s more, extreme weather events like  floods are likely to become more frequent as the effects of climate change take hold. As we saw in our first episode,   that increases the risk of creating  reservoirs of water-born pathogens which lead to outbreaks of disease like cholera  and parasitic infections like Cryptosporidium, which causes diarrhea.

And high-income countries aren’t  free from those risks either! A flood in Germany in 2013 caused an  outbreak of Cryptosporidium infections too. So like the social and economic kind, in reality environmental factors  pose global risks for outbreaks.

It’s just that, as we’ve seen, those risks are often higher for less wealthy  and less socially supported groups than others. And we’ve started to see how  all of this is connected. This is especially evident in a big,  history-shifting event like war.

Wars are often driven by social differences and  economic inequalities between different groups, which spark into open conflict. Conflict and the mass displacement of people that  follows often puts a strain on the environment. When groups of people enter into conflicts it  can increase the likelihood of an outbreak.

The most straightforward case would be  when the movement of infected soldiers spreads disease across communities. But as we saw in the last episode, conflict also destroys physical infrastructure  that keeps neighborhoods safe and clean, increasing the risk of  being infected by pathogens. In addition, millions of people can be  forced to leave their homes to escape   the violence and disruptions to their way of life, becoming refugees.

Refugees often end up in crowded, makeshift camps  which make hotbeds for the spread of disease. For example, in 2013 refugees from the war in  Syria experienced an outbreak of Hepatitis A, a virus that leads to inflammation in the liver, because of the poor sanitary  conditions in their camp in Lebanon. While the social conditions helped the   refugees escape conflict because of  Lebanon’s border policy with Syria, economic insecurity, which prevented  mass vaccination strategies, and environmental conditions like  pathogen-carrying groundwater, all contributed to the outbreak.

In the next episode, we’ll look at another  way that outbreaks are connected to conflict when we talk about the weaponization of outbreaks. That all sounds… a little bleak. But being aware of how these factors  influence the risk of outbreaks and for who is a vital starting point for coordinating  the world’s efforts to fight back.

Later in this series we’ll be looking at  some strategies for doing exactly that. We at Crash Course and our partners Operation Outbreak and the Sabeti Lab at the Broad Institute at MIT and Harvard want to acknowledge the Indigenous people native to the land we live and work on, and their traditional and ongoing relationship with this land. We encourage you to learn about the history of the place you call home through resources like and by engaging with your local Indigenous and Aboriginal nations through the websites and resources they provide.

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.