A typical virus is only about 100 billionths of a meter across.
If all of the viruses on earth were laid end-to-end, how far do you think that chain of germs would
stretch?
Tens of millions of meters?
Tens of millions of kilometers?
The answer is tens of millions of light years.
There are a LOT of viruses out there.
And that is a scary thought.
But!
Viruses are cellular parasites.
They can't survive unless they are inside the cells of some other living thing.
And without these cells to call home, that chain of germs would 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, and making us
very, very sick.
Why?
The answer to that tells us something very important about our place in nature.
And it makes one thing really clear: Viruses aren’t out to get us.
We go out and get them.
[MUSIC]
Hey smart people, Joe here.
Over the past few months, everyone’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 when we take a moment to understand why, when, and how that happens, 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 this book, “Spillover: Animal infections and the Next Human Pandemic”
This is a book I think about a lot, which is why it’s on the shelf behind me in pretty
much every other video 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 he was 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 said, “Oh, that’s quaint: animal
infections and 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,
that’s what’s going to 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 global pandemic that has infected millions
and killed hundreds of thousands 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 going to take a look at why and how
these things even happen.
And it turns out they are not new.
As variously estimated, scientists say 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!
Scientists today know that from comparing genetic sequences from germs taken from wild
species with germs that infect us.
And they see similarities to tell us 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 titled “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 plague 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 an only human disease.
It comes from some sort of a wild morbililvirus and diverged from an animal virus maybe 4th
century BC.
Other estimates maybe say 9th century AD.
Our smallpox was a divergent strain that had gotten into humans from animals, a long, long
time ago, tens of thousands of years ago.
Maybe what we now call a cowpox, or a horsepox.
Then it stayed in humans long enough, and it was evolving fairly quickly.
So it diverged and became uniquely a human virus.
Now, smallpox is special for another reason: It happens to be the only infectious disease
we’ve ever completely eradicated.
And that was only possible because the smallpox virus that infects humans isn’t out there
hiding in another animal species, waiting to jump into humans again.
And that brings us to the first ingredient 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 it just quietly hangs out inside 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 these attempts almost always fail, because the two hosts are just too different.
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 billions.
So they can develop a lot of mutations really fast.
Many of these mutations don't do anything, and many 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.
That virus opens up a whole new universe where it can make more of itself, and in the game
of evolution, that's the only prize that matters.
And if I – the virus – can do that, I’m on my way to a new phase of evolutionary success,
a more ambitious phase than what I had when I was living in low concentrations in my reservoir
host.
I was keeping it 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 time.
Then you perpetuate yourself.
That’s the survival of the fittest.
That’s what viruses as well as people, and dandelions, and rats do.
And the closer two hosts are on the evolutionary tree, the fewer mutations a germ needs to
let it make that leap.
A virus adapted to a close relative of ours, say chimpanzees, might not need many mutations
at all.
Scientists now think HIV made a leap just like this, from chimpanzees into humans, sometime
early in the 20th century, when a human hunted or came in 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, every
ecosystem.
Wild animals, they're trading viruses all the time.
It’s not just wild animals, nonhuman animals, sending their viruses to us, downloading their
viruses onto us, dumping on us.
Viruses are moving every which way, all the time.
Because all wild animals carry viruses, they carry a 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 spillover 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.
Thing is, 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 bring wild things close to
us.
The wording that people use is that an animal has “caused” a disease in us.
I'm sensing that doesn’t really line up with how these spillovers are 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 that’s
a great opportunity.
The virus falls into us, we bring the virus into our cells.
By bringing these wild animals closer to us.
But even if all these ingredients are there: a virus mutates in a reservoir host, it wins
the virus lottery and comes into contact with one human, and it’s able to make the leap
of infection into 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 bioweapon, they might build an aerial vehicle
that you could fill with infection, and then get to every major city in the world within
hours, where those containers would be opened up and that infection would be dispersed into
unsuspecting crowds.
In other words, you’d invent air travel.
Now, I’m not saying 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 of how the tools that connect us also connect our germs.
It’s an irony that we humans are closely connected, 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 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 ingredient of a zoonotic pandemic.
The human species is reaching into the wild everywhere we can, and pulling out germs.
We're pouring fuel on the fire of virus evolution
There’s SARS CoV-3 and SARS Co-V-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 We have a tendency to think of ourselves as separate from the natural world.
There’s humans, and the human world.
And then there’s the natural world over there - maybe you go there on Sunday to take
a hike.
But there is no this world and 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 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.
There’s something that David wrote that’s stuck with me.
He wrote this about Ebola, another zoonotic disease, while he was hiking through the jungles
of 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 its habitat.
Stay curious.
Finally I just want to say a huge thank you to everyone who supports the show on Patreon.
Like so many other people, COVID-19 has completely changed the way that we work.
But I just feel very very lucky that we are still able to bring you these videos and hang
out together and learn something along the way.
And that is thanks to your support.
If you'd like to join our community, there's a link down in the description where you can
learn more and all the great other stuff.
And if not, just happy that you're here watching this video.
See you next time.