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Our bodies contain all sorts of microscopic organisms, like bacteria and viruses.
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Some of these are pathogens that can cause disease.
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Animals' bodies have them too.
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When a pathogen jumps from one species, to another species that isn't familiar with it,
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it can exploit that new host's lack of defenses, and cause illness.
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A pathogen that moves from animals to humans is called a "zoonosis."
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Diseases like West Nile virus and Ebola both originated this way.
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And researchers think Covid-19 did too.
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Among humans, the majority of new disease outbreaks are the result of zoonotic diseases.
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And for the last few decades, the number of zoonotic disease outbreaks has been increasing.
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Looking at this chart, it might seem like humans are the victims
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of an onslaught of pathogens from animals.
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But what if these outbreaks are increasing because of something humans are doing?
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There's a lot of things we're doing that are increasing the probability
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of pandemic-causing pathogens emerging.
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This is Sonia Shah. She's a science journalist who writes about the history of pandemics.
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Right now we've used up over half of the terrestrial surface of the planet.
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Humans have been using more and more land for hundreds of years,
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but that land use has accelerated in the last hundred years.
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Today, satellite imagery shows us exactly what this expansion looks like:
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We're expanding our cities,
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erasing forests,
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and reshaping the land for agriculture.
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And sometimes that expansion is the result of war.
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That's what happened here, in West Africa.
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It's an area that, today, includes Sierra Leone, Guinea, and Liberia.
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There was a forest that once covered that area where the three countries meet.
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Throughout the 1990s, civil wars in this region killed thousands
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and forced many more from their homes.
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Hundreds of thousands of refugees fled into that forest. They cut down a lot of those trees.
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They were making way for their homes and cutting down trees
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for charcoal and farming, et cetera.
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And you can actually see the change in the forest cover in satellite images.
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If you look at the early 70s satellite images, it's almost all green.
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And then you look at late 1990s, and it's mostly brown,
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because only a small fraction of the original forest remained.
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Here, in a village in Guinea, is where researchers think an outbreak of Ebola began,
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at the end of 2013.
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Like the rest of the region, this village used to be a forest, and also happens to be
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a natural habitat for wild bats.
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Researchers believe the outbreak started with a boy named Emile,
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who died after he was exposed to fluids from a bat carrying Ebola,
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which quickly spread from his family,
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to his village, to other villages.
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As humans develop and transform wild animal habitats, events like the Ebola outbreak are
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becoming more and more likely.
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We're paving over wildlife habitat, which means it's much more likely that pathogens
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that live inside animal bodies will make their way into human bodies.
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And that's because animals end up living much closer to us.
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But in many cases, those animals don't survive human encroachment into their habitats in the first place.
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This chart shows the biggest extinction threats facing different animal groups.
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For most of them, the biggest threat to survival isn't pollution, or being hunted —
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it's the loss of their habitat.
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And a disappearing species creates a different opportunity for zoonotic diseases to jump to humans.
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Take West Nile virus, for example.
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It originates in birds that migrate from Africa to North America in the summer.
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It typically infects humans through mosquitos.
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But West Nile virus was never a problem in the US.
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Until 1999.
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That's because we had a diversity of bird species in our domestic bird flocks.
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In a diverse bird population, some species are good carriers of West Nile virus,
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and some aren't.
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North America once had a large population of birds like woodpeckers, and rails,
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that don't easily carry the virus.
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That made it much harder for the virus to spread among the bird population.
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But then we started disrupting those birds' habitats.
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What's happened over the past 50 years is, we've lost a lot of that avian biodiversity.
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Woodpeckers and rails are now pretty rare.
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What we now have instead are a lot of species like American robins and crows.
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Crows and robins are much more adaptable to a changed environment.
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But they also happen to be better carriers of West Nile virus.
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The fewer woodpeckers and rails you have around and the more robins and crows you have around,
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the more West Nile virus you have around in your domestic bird flocks.
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And the more likely it becomes that a mosquito will bite an infected bird and then bite a human.
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And that's exactly what happened in New York City in 1999.
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Before 1999, no one in the US had ever died from West Nile virus.
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Since then, around 150 people in the US have died from it every year.
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The places where humans are encroaching on wildlife are the frontier for the next pandemic.
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That means one thing scientists can do to prevent it, is to watch those places really closely.
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We don't know which microbe is going to cause the next outbreak or pandemic, but we do know
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how that happens. And so we can really do active surveillance in those places where
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it's most likely to occur, places where there's a lot of invasion of wildlife habitat.
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But preventing future outbreaks might also require us to rethink our relationship with nature,
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and to understand that, as we take over more and more of the planet, there's a cost —
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to the animals that live there,
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but also, to us.