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!
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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 mosquitos.
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 native-land.ca
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.