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  • Our bodies contain all sorts of microscopic organisms, like bacteria and viruses.

  • Some of these are pathogens that can cause disease.

  • Animals' bodies have them too.

  • When a pathogen jumps from one species, to another species that isn't familiar with it,

  • it can exploit that new host's lack of defenses, and cause illness.

  • A pathogen that moves from animals to humans is called a "zoonosis."

  • Diseases like West Nile virus and Ebola both originated this way.

  • And researchers think Covid-19 did too.

  • Among humans, the majority of new disease outbreaks are the result of zoonotic diseases.

  • And for the last few decades, the number of zoonotic disease outbreaks has been increasing.

  • Looking at this chart, it might seem like humans are the victims

  • of an onslaught of pathogens from animals.

  • But what if these outbreaks are increasing because of something humans are doing?

  • There's a lot of things we're doing that are increasing the probability

  • of pandemic-causing pathogens emerging.

  • This is Sonia Shah. She's a science journalist who writes about the history of pandemics.

  • Right now we've used up over half of the terrestrial surface of the planet.

  • Humans have been using more and more land for hundreds of years,

  • but that land use has accelerated in the last hundred years.

  • Today, satellite imagery shows us exactly what this expansion looks like:

  • We're expanding our cities,

  • erasing forests,

  • and reshaping the land for agriculture.

  • And sometimes that expansion is the result of war.

  • That's what happened here, in West Africa.

  • It's an area that, today, includes Sierra Leone, Guinea, and Liberia.

  • There was a forest that once covered that area where the three countries meet.

  • Throughout the 1990s, civil wars in this region killed thousands

  • and forced many more from their homes.

  • Hundreds of thousands of refugees fled into that forest. They cut down a lot of those trees.

  • They were making way for their homes and cutting down trees

  • for charcoal and farming, et cetera.

  • And you can actually see the change in the forest cover in satellite images.

  • If you look at the early 70s satellite images, it's almost all green.

  • And then you look at late 1990s, and it's mostly brown,

  • because only a small fraction of the original forest remained.

  • Here, in a village in Guinea, is where researchers think an outbreak of Ebola began,

  • at the end of 2013.

  • Like the rest of the region, this village used to be a forest, and also happens to be

  • a natural habitat for wild bats.

  • Researchers believe the outbreak started with a boy named Emile,

  • who died after he was exposed to fluids from a bat carrying Ebola,

  • which quickly spread from his family,

  • to his village, to other villages.

  • As humans develop and transform wild animal habitats, events like the Ebola outbreak are

  • becoming more and more likely.

  • We're paving over wildlife habitat, which means it's much more likely that pathogens

  • that live inside animal bodies will make their way into human bodies.

  • And that's because animals end up living much closer to us.

  • But in many cases, those animals don't survive human encroachment into their habitats in the first place.

  • This chart shows the biggest extinction threats facing different animal groups.

  • For most of them, the biggest threat to survival isn't pollution, or being hunted

  • it's the loss of their habitat.

  • And a disappearing species creates a different opportunity for zoonotic diseases to jump to humans.

  • Take West Nile virus, for example.

  • It originates in birds that migrate from Africa to North America in the summer.

  • It typically infects humans through mosquitos.

  • But West Nile virus was never a problem in the US.

  • Until 1999.

  • That's because we had a diversity of bird species in our domestic bird flocks.

  • In a diverse bird population, some species are good carriers of West Nile virus,

  • and some aren't.

  • North America once had a large population of birds like woodpeckers, and rails,

  • that don't easily carry the virus.

  • That made it much harder for the virus to spread among the bird population.

  • But then we started disrupting those birds' habitats.

  • What's happened over the past 50 years is, we've lost a lot of that avian biodiversity.

  • Woodpeckers and rails are now pretty rare.

  • What we now have instead are a lot of species like American robins and crows.

  • Crows and robins are much more adaptable to a changed environment.

  • But they also happen to be better carriers of West Nile virus.

  • The fewer woodpeckers and rails you have around and the more robins and crows you have around,

  • the more West Nile virus you have around in your domestic bird flocks.

  • And the more likely it becomes that a mosquito will bite an infected bird and then bite a human.

  • And that's exactly what happened in New York City in 1999.

  • Before 1999, no one in the US had ever died from West Nile virus.

  • Since then, around 150 people in the US have died from it every year.

  • The places where humans are encroaching on wildlife are the frontier for the next pandemic.

  • That means one thing scientists can do to prevent it, is to watch those places really closely.

  • We don't know which microbe is going to cause the next outbreak or pandemic, but we do know

  • how that happens. And so we can really do active surveillance in those places where

  • it's most likely to occur, places where there's a lot of invasion of wildlife habitat.

  • But preventing future outbreaks might also require us to rethink our relationship with nature,

  • and to understand that, as we take over more and more of the planet, there's a cost

  • to the animals that live there,

  • but also, to us.

Our bodies contain all sorts of microscopic organisms, like bacteria and viruses.

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B1 Vox nile west bird ebola habitat

How humans are making pandemics more likely

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    林宜悉 posted on 2020/07/28
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