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.