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.
