(soft music) - The typical virus is only 100 billionths of a meter across, so if you laid all the viruses on earth end to end, how far do you think that chain of germs would stretch?

Tens of millions of meters?

Tens of millions of kilometers?

Well, the answer is tens of millions of light years.

There are a lot of viruses out there.

That's kind of a scary thought.

But viruses are cellular parasites, which means they can't survive unless they're inside the cells of some other living thing.

Without those other cells to call home, that chain of viruses would just disappear.

Plants, fungi, animals, every living species on earth today is home to its own universe of viruses.

But more and more, viruses are jumping out of other animals and into us, making us very, very sick along the way.

Why?

The answer to that teaches us something very important about our place in nature and it also makes one thing really clear, viruses aren't out to get us, we go out and get them.

(light upbeat music) Hey, smart people, Joe here.

You know, over the past few months, everybody's been talking about what's happening right now with SARS-CoV-2, but SARS-CoV-2 is just the latest chapter in a long story of viruses jumping from one animal species and into our own, what we call zoonotic infections.

And if we take a moment to understand why, when, and how these happen, it can teach us some really important lessons about how to avoid SARS-CoV-3, or whatever the next one is, because if we don't change anything, experts are 100% certain this will happen again.

So speaking of experts, I called one up, science writer, David Quammen.

In 2012, he wrote a book called "Spillover: Animal Infections and the Next Human Pandemic."

This is a book that I think about a lot which is why it's on the shelf behind me in pretty much every other video that I make.

David's book basically saw all of this coming, a fast mutating respiratory virus that jumped out of a mammal in Asia into homo sapiens, causing a massive global pandemic.

So I asked him, "How was he able to make these predictions so many years ago?"

When things get really bad and people start getting sick around the world, everyone seems to call you.

- My book contains pretty precise predictions of what is happening now, not because I was prescient, but because I was listening to a carefully selected group of disease scientists and public health people.

When I published it back in 2012, a lot of people sort of said, "Well, oh, well, that's quaint, animal infections in the next human pandemic, that must be this sort of fringe subject out on the edge of medicine."

That was the reaction then, except among a certain number of people who said, "Yeah, yeah, that is what's gonna happen."

- And just like experts predicted, it did happen.

SARS-CoV-2 jumped out of another species, most likely a bat, maybe passing through another species in between, and into humans, causing a massive global pandemic that's infected millions and killed hundreds of thousands of people in just months.

Zoonotic diseases are definitely no longer a fringe subject on the edge of medicine.

So now that the world is paying attention, we're gonna take a look at why and how these things happen, and it turns out, they are not new.

- As variously estimated, scientists say that 60 to 70% of human infectious diseases are zoonotic diseases, coming from non-human animals, spilling into humans.

- I was completely shocked when I heard that number.

Most human infectious diseases originally came from other animals, 60 to 70% of them, and scientists today know that from comparing genetic sequences from germs taken from wild species, with germs that infect us, and they see enough similarities to tell us that many human germs are the distant offspring of those that we find in wild species.

They somehow made the leap into us.

- I have a chapter in "Spillover" entitled, "Everything Comes from Somewhere."

And when you think about it, we're a relatively young species, they reckon 200,000 years, so we're here and we have infectious diseases.

Where could those infectious diseases come from, except from other animals?

- Think about that.

Bubonic playing or COVID-19, every disease that we get had to come from somewhere.

And even for germs that have called us home for millennia, that somewhere is most likely other animals.

- Measles is considered only a human disease, comes from some sort of a wild morbillivirus virus and diverged from an animal virus, maybe the 4th century BC.

Other estimates say maybe 9th century AD.

Our smallpox was a divergent strain, had gotten into humans from animals long, long time ago, thousands, tens of thousands of years ago, maybe what we now call a cowpox or a horsepox, and then it stayed in humans long enough that, and it was evolving fairly quickly, and so it diverged and it became a uniquely human virus.

- Now smallpox is special for another reason because it happens to me the only infectious disease that we've ever completely eradicated and that was only possible because the smallpox virus that infects humans isn't out there hiding in some other animal species waiting to jump into humans again.

And that brings us to the first ingredient that a germ needs in order to make a species jump, a reservoir host.

Animals in the wild are constantly getting infected with viruses, but those animals don't always get sick because if a virus is too successful, it just runs out of hosts, so instead, sometimes just quietly hangs out inside of a reservoir species.

This is one reason there's such an unfathomable number of viruses out there, enough to stretch across our galaxy and beyond.

And those viruses are constantly encountering new hosts and trying to infect them, but most of the time they fail because the two hosts are just two different.

I mean, for a virus that's adapted to infecting fish, the human respiratory tract might as well be another planet.

But when viruses reproduce, they do it by the millions or the billions so they can develop a lot of mutations really fast.

Now, many of those mutations don't do anything and many of them actually make the virus worse, but every once in a while, a virus randomly rolls the mutation dice and wins the jackpot, the ability to infect a new host, and that virus opens up an entirely new universe where it can make more of itself, and in the game of evolution, that's the only prize that matters.

- If I can do that, if I, the virus, can do that, now I'm on my way toward a new phase of evolutionary success, a more ambitious phase than what I had when I was living at low concentrations in my reservoir host.

I was living, keeping it sort of on the down-low.

I wasn't making a lot of trouble, but the evolutionary mandates, replicate yourself as much as possible, as quickly as possible, and extend yourself in space and in time, then you perpetuate yourself and that's the survival of the fittest.

That's what viruses, as well as people and dandelions and rats do.

- And the closer that two hosts are on the evolutionary tree, the fewer mutations a germ needs in order to make that leap.

A virus that's adapted to a close relative of ours, say chimpanzees, might not need very many mutations at all.

And scientists now think that HIV once made a leap just like this from chimpanzees into humans sometime in the early 20th century when a human hunted or came into contact with blood from an infected chimp, and this brings us to the next ingredient for a spillover: contact.

The churning and cooking of evolution is always happening everywhere, in every organism, in every ecosystem.

- Wild animals, they're trading viruses all the time.

So it's not just, you know, wild animals, non-human animals sending their viruses to us, downloading their viruses onto us, dumping on us.

Viruses are moving every which way all the time 'cause all wild animals carry viruses and they carry a great diversity of viruses.

Bats, for instance, carry a lot of viruses, including a great diversity of coronaviruses.

It's important to say these viruses don't want to spill over from bats or rodents into humans.

They're not after us.

We simply present ourselves as an incredible opportunity to them.

- Even now, there could be a virus hiding out in tigers in Siberia that could infect rabbits in Australia.

The thing is, those species will never come into contact.

But the human species is in more places, interacting with more wild stuff than any other species on earth when we destroy those wild places or we bring those wild things close to us.

The wording that people use about this is an animal has caused a disease in us.

I'm sensing that doesn't really line up with how these spillovers are really happening.

- There's nothing special about us.

There's just more of us.

We're the world's biggest target for viruses, but we're not the only target.

Every time we come in contact with a wild animal, we offer ourselves as an opportunity for new possibilities, a new host.

And as I said, the virus, doesn't jump into us.

It doesn't look at us and say, "Wow, there's a great opportunity."

The virus falls into us.

We bring the virus into ourselves by bringing these wild animals closer to us.

- But even if all of these ingredients are there, a virus mutates in a reservoir host, if it wins the virus lottery and comes into contact with one human and it's able to make the leap of infection into that one person, well, that's still not enough.

To spread, a germ needs to be able to transmit between people because two new hosts means double the chance to mutate and evolve into an even more successful virus and so on and so on.

If someone was going to design the perfect bio weapon, they might make an aerial vehicle that you could fill with infection and then get to every major city on earth within a few hours where those containers would be opened up that infection would be dispersed into unsuspecting crowds.

In other words, you'd invent air travel.

Now I'm not saying that airplanes are bad.

It's totally amazing that we're intelligent enough to build flying machines that can connect every corner of the planet, but I'm using it as an example that the tools that connect us also connect our germs.

- It's an irony that, you know, we humans are closely connected and that's both an advantage and a disadvantage.

We need to be closely connected because we need scientific information to be passing at the speed of light around the world so that we can be prepared for the next one.

But the downside is there are these drone-like machines that carry viruses all over the world and drop them into cities.

- We are the final ingredients of a zoonotic pandemic.

The human species is reaching into the wild everywhere that we can and pulling out germs.

We're pouring fuel on the fire to accelerate virus evolution.

- There's SARS-CoV-3 and SARS-CoV-4 out there, so yes, we have to change our behavior or there will be more versions of this spilling over into humans, causing outbreaks, if not causing pandemics.

- Does this give us an opportunity to become aware of our place in nature in a different way?

- Absolutely it does that.

We have a tendency to think of ourselves as separate from the natural world.

There's humans in the human world and then there's the natural world over there.

Yeah, maybe you go there on Sunday to take a hike, but there is no "this world in that world," there's just the world.

One of the scary things that Darwin said to the world in 1859 was that we humans are animals.

What's a good really good reminder of that?

We share diseases with animals.

A virus that infects a bat can infect us too.

It happens because we're mammals like them.

Animal disease, human disease, same disease.

- Now there's something that David wrote that has stuck with me.

He wrote this about ebola, another zoonotic disease, after he was hiking through the jungles in Central Africa a few miles from an outbreak, but it applies here and it applies to our future.

"The virus is not in your habitat.

You are in its."

- I knew that ebola was there.

It was everywhere and nowhere.

Ebola is not everywhere on the planet right now, but this novel coronavirus is.

It's here, it's among us.

- We're in its habitat and it's time to think like that.

- We are it's habitat.

- Stay curious.