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You are part of one of the world’s greatest
endeavors: the effort to stop infectious disease.
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Like other animals, infectious diseases have
been with us since the dawn of our species,
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from herpes virus that infected our common
ancestors in Africa
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to the Covid-19 pandemic that struck the whole
world.
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The effort to understand these diseases has
been going on for some time, too.
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As far back as 400 BCE, civilisations in different
parts of the world tried to work out
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who became ill and why.
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Since then, we’ve come a long way in understanding
what diseases are and how they spread.
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And that knowledge is vital to stopping them.
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But it’s not only doctors and scientists
who can act on what we know.
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Infections involve individual people,
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so everything from the way you and I go about
our everyday activities
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to the way we organise whole societies all
influence how an outbreak evolves –
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and whether we overcome it.
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I’m Pardis Sabeti, a professor of genetics
at Harvard University,
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where I study infectious diseases.
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In this series, we’re going to take a deeper
look at disease outbreaks,
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from the microbiology and genetic factors
behind them to healthcare systems,
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social structures and people who tackle them,
including you!
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Welcome to Crash Course Outbreak Science!
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[Theme Music]
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The study of outbreaks is a little different
from your garden variety science.
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Not everything about an outbreak can be studied
experimentally.
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Outbreaks are complex situations involving
real people in their environments,
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so we can’t perfectly measure and control
all the variables.
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It would also, you know, be highly unethical
to release an infectious disease into a population
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just to study what happens –
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though, sadly, that hasn’t stopped people
from doing just that.
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Even so, it helps to be specific about what
we’re talking about when using words like
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“epidemics” and “outbreaks”.
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The American Public Health Association’s
Control of Communicable Diseases Manual
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– yes, there’s a handbook for this! –
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defines an epidemic as:
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“The occurrence, in a defined community
or region, of cases of an illness
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(or an outbreak)
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with a frequency clearly in excess of normal
expectancy.”
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In other words, an epidemic is when many more
people in a group than usual develop a particular illness.
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We can also use the word “outbreak”, usually
when the region we’re talking about is relatively small.
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That might all seem straightforward, but there’s
a question hidden here.
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What counts as a “usual” amount of illness?
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It turns out, it depends on the community.
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For example, in the 1990s there was an outbreak
of cholera in Latin America.
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Cholera is a bacterial disease that attacks
the intestines,
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usually contracted from contaminated water,
and nearly a million people were infected.
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Those kinds of numbers for cholera cases hadn’t
been seen on the continent in over a century,
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so the sudden rise was dramatic.
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But near the Ganges delta between Bangladesh
and India,
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cases of cholera are an unfortunate and persistent
fact of life because of the environment.
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When a disease has roughly the same incidence
in a place over an extended period of time,
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it’s called endemic.
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The word endemic is derived from the ancient
greek words “en” meaning “in”
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and “demos” meaning “people”, like,
diseases that reside in a group of people.
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That’s opposed to “epidemic”, which
is derived from “epi” meaning “on”,
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as in, diseases that act on a group of people.
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Similarly a pan-demic draws on the greek word
“pan” meaning “all”,
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as in an epidemic that spreads across borders
into many countries,
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affecting the whole world if left unchecked.
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What really distinguishes an epi-demic, or
outbreak,
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is that the number of cases of illness are
much higher than they normally are within
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a certain community.
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Which means words like “normal” and “usual”
have to be taken in the context of particular groups of people,
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their way of life, and circumstances.
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It doesn’t mean that the cases of cholera
were more or less important in South America
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than in the Ganges Delta.
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But it means we might approach the two situations
differently.
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Outbreaks are evolving situations where the
disease could become more widespread.
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So the goal is to stop the disease spreading
even further, as well as treating those who have it.
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It also means having organizations and systems
in place that can handle the social changes
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an outbreak requires.
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Which is a pretty tall order!
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What’s key to tackling those challenges
is understanding the nature of an infectious
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disease and how people respond to them.
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We’ll start with the disease itself.
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When it comes to diseases, different scientists
have different ways of thinking about them.
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For example, microbiologists consider diseases
in terms of their biological cause.
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One of the most important scientific discoveries
in history is that tiny organisms that enter the human body
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are what’s often responsible for us contracting
infectious diseases.
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These are known as pathogens.
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The most commonly known pathogens are microorganisms
like bacteria, viruses, protozoa and fungi.
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The science behind pathogens is pretty broad,
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so we’ll be looking at them in closer detail
next episode!
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The take-away is that microbiologists tend
to characterize diseases by the pathogens that cause them.
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Epidemiologists meanwhile think about diseases
in terms of the bigger picture,
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focusing on how it spreads and what its source
is.
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Unfortunately for us, there are lots of ways
pathogens can infect people.
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They might be spread from person to person
through contact with skin
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or through droplets in the air from someone’s
sneezes or coughs.
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They might be eaten in contaminated food or
injected by an insect like a mosquito.
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Even if the pathogens might be different,
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an epidemiologist would think of diseases
in terms of these transmission routes.
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They also consider when a community is regularly
coming into contact with the same source of
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disease, which are called reservoirs.
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Sometimes, a reservoir is a group of infected
people,
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but it could also mean a local population
of animals carrying diseases like rabies.
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The reservoir might be soil which contains
certain kinds of fungi or contaminated water.
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The kind of reservoir also determines who
might be at the most risk from an outbreak,
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as we’ll see in a moment.
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Finally, there are medical scientists and
doctors,
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who tend to think of diseases by their clinical
symptoms.
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Clinical symptoms might be familiar things
like fever or difficulty breathing but also
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could be conditions like, say,
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inflammation and swelling of body parts or
a whole lot of other not-so-fun things.
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From a doctor's perspective, identifying diseases
by their symptoms is important for treating them.
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It’s worth mentioning we’re going to have
to be frank about the human body
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and some of the more icky parts when studying outbreaks.
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But science demands clarity!
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For example, in the case of diarrhea, regardless
of the pathogen or the way the patient was infected,
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they need to be treated with fluid replacement
to keep them hydrated.
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So we have three ways of looking at disease:
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the organism behind the disease, the way it
spreads, and how it affects an infected person.
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Which brings us to the role people play in
an outbreak.
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Different perspectives of disease help inform
groups like healthcare providers,
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public health experts and the communities
themselves to tackle outbreaks.
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To see how this all comes together, let’s
look at a real life example.
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We’ve mentioned that cholera is endemic
to the Ganges Delta,
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just off the coast of West Bengal in India.
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Although it’s endemic in the whole region,
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scientists can still identify outbreaks in
a given community,
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exceeding their expected number of cases.
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Let’s go to the Thought Bubble.
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On October 13, 2004, a healthcare facility
in Kanchrapara, India,
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reported a cluster of cases where patients
suffered from acute, watery diarrhea.
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As I said, science needs clarity!
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A lot of those patients also were so dehydrated
they were sent to the hospital —
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all classic signs of cholera infection.
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The district epidemiologist set about to confirm
whether this was an outbreak.
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He started by looking at the data of similar
diarrhea cases from the previous months
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and found that the number of cases in the
cluster was in fact higher than expected.
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To confirm that the patients definitely had
cholera, he worked with the hospital to take samples,
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which, for clarity, were from the patients’...
butts.
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The samples were from their butts.
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Those samples were sent to a lab, where microbiologists
could test them and rule out other pathogens
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that might be responsible too, like salmonella.
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Meanwhile, the epidemiologist drew a map of
the cases by household.
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He found that most of the cases came from
areas which relied on the municipal water supply.
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In fact, a nearby area that used a different
water supply had fewer cases.
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It turned out that earlier that same month,
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the municipal water supply had sprung a leak
in its pipeline.
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That was a major clue.
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A leak would make it possible for fluids to
be sucked into the pipeline and contaminate the water.
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What’s more, it had been raining heavily
at the time,
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bringing lots of sewage-contaminated water
near the pipeline.
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Sure enough, the lab results came back positive
for Vibrio cholerae,
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the bacteria that causes cholera, and negative
for other pathogens.
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At that point, it was clear there was a cholera
outbreak on their hands.
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Thanks, Thought Bubble!
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A later environmental assessment with the
city’s engineers found that, as suspected,
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the leak in the pipeline had sucked up sewage-contaminated
water into the water supply.
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Thankfully, by that point the leak had been
repaired and water had been chlorinated.
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And shortly after the intervention, the number
of new cholera cases had fallen rapidly!
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We can see how different perspectives on disease
helped identify and resolve the outbreak.
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The clinicians monitoring clinical symptoms
helped bring the high number of diarrhea cases to light
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and flag up the possibility of cholera.
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Microbiologists confirmed this by identifying
the pathogen from lab testing,
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while epidemiologists identified the reservoir
for the disease.
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While outbreaks are specific to certain groups
of people,
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the way in which they happen is often starkly
similar to outbreaks all over the world.
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While we’ve mentioned cholera outbreaks
in Latin America and India so far,
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one of the most famous ones happened 150 years
earlier in London, England.
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Though the tools at his disposal were a little
different in 1854,
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physician John Snow used many of the same
methods that the district epidemiologist in
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Kanchrapara used to identify the outbreak.
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Snow also used a map to trace the locations
of each cholera case to find the common source of infection.
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Turns out, a contaminated water pipe was the
culprit of that outbreak, too.
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Contemporary outbreak scientists still map
all kinds of outbreaks,
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often with advanced geospatial techniques
and software,
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including NASA earth observing research satellites.
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And in both Kanchrapara and London,
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the reservoir also highlighted who was susceptible
to certain kinds of outbreaks.
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It was clear that those who relied on the
municipal water supply were already exposed
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to risks from the unfit water system, even
before the outbreak.
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And we’ll see throughout the series,
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environmental conditions play a huge role
in determining how often outbreaks occur and
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who is affected by them.
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But people and practices were also at the
heart of the outbreak response.
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In Kanchrapara, monitoring and collecting
data from healthcare facilities
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required social practices that encouraged
reporting unusual scenarios, like a cluster of symptoms.
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There were also organizational links between
healthcare facilities and epidemiologists
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that made sure information was flowing in
a useful way.
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Transporting the samples and having laboratories
equipped to analyse them required locally
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available technology and infrastructure.
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Even before the outbreak, decisions had been
made to have an epidemiologist in the area
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and labs with the capacity to test for cholera
in the first place!
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After the outbreak, scientists worked with
hospital clinicians, city water engineers,
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district authorities and the chief medical
officer to plan for next steps.
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That included an investigation of the city
pipelines for leaks so they could be fixed
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before the next outbreak
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and making sure the water was chlorinated
from that point on to prevent cholera infecting the supply.
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Which brings us to a final point about tackling
outbreaks: communication.
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Different groups, from patients, scientists,
governments and public health workers,
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need to share information and collaborate
during an outbreak to ensure the right steps are taken.
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Throughout this series, we’ll continue to
look at how the way people interact with one another
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and the social structures they inhabit all
play a role in how outbreaks develop and how we can stop them.
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And hopefully, what we learn in this series
will enable you to play a role too.
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Next time, we’ll get into the microscopic
world of pathogens.
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See you then!
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We at Crash Course and our partners Operation
Outbreak
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and the Sabeti Lab at the Broad Institute
at MIT and Harvard
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want to acknowledge the Indigenous people
native to the land we live and work on,
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and their traditional and ongoing relationship
with this land.
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We encourage you to learn about the history
of the place you call home through resources
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like native-land.ca and by engaging with your
local Indigenous and Aboriginal nations
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through the websites and resources they provide.
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Thanks for watching this episode of Crash
Course Outbreak Science,
00:11:10
which was produced by Complexly in partnership
with Operation Outbreak
00:11:13
and the Sabeti Lab at the Broad Institute
of MIT and Harvard—
00:11:17
with generous support from the Gordon and
Betty Moore Foundation.
00:11:20
If you want to help keep Crash Course free
for everyone, forever,
00:11:23
you can join our community on Patreon.
