Why Do Deadly Viruses Jump From Animals to Humans?


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 wouldstretch?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 universeof viruses. But more and more, viruses are jumping out
of other animals and into us, and making usvery, 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. Hey smart people, Joe here. Over the past few months, everyone’s been
talking about what’s happening right nowwith SARS-CoV-2. But SARS-CoV-2 is just the latest chapter
in a long story of viruses jumping from oneanimal 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 ussome really important lessons about how to
avoid SARS-CoV-3 or whatever the next oneis… because if we don’t change anything,
experts are 100% certain this will happenagain. 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 prettymuch every other video I make. David’s book basically saw all of this coming:
A fast mutating respiratory virus that jumpedout 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 seemsto call you. My book contains pretty precise predictions
of what is happening now, not because I wasprescient, but because I was listening to
a carefully selected group of disease scientistsand public health people. When I published it back in 2012, a lot of
people said, “Oh, that’s quaint: animalinfections 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 anotherspecies in between, and into humans, causing
a global pandemic that has infected millionsand 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 howthese things even happen. And it turns out they are not new. As variously estimated, scientists say 60
to 70% of human infectious diseases are zoonoticdiseases – 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 wildspecies with germs that infect us. And they see similarities to tell us many
human germs are the distant offspring of thosethat 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 mostlikely 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 4thcentury BC. Other estimates maybe say 9th century AD. Our smallpox was a divergent strain that had
gotten into humans from animals, a long, longtime 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 diseasewe’ve ever completely eradicated. And that was only possible because the smallpox
virus that infects humans isn’t out therehiding 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’talways 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 stretchacross 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 aswell 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, butevery once in a while a virus randomly rolls
the mutation dice and wins the jackpot: theability to infect a new host. That virus opens up a whole new universe where
it can make more of itself, and in the gameof 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 reservoirhost. 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 tolet it make that leap. A virus adapted to a close relative of ours,
say chimpanzees, might not need many mutationsat all. Scientists now think HIV made a leap just
like this, from chimpanzees into humans, sometimeearly in the 20th century, when a human hunted
or came in contact with blood from an infectedchimp. 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, everyecosystem. Wild animals, they’re trading viruses all
the time. It’s not just wild animals, nonhuman animals,
sending their viruses to us, downloading theirviruses 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 intohumans. 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 rabbitsin 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 anyother species on Earth, when we destroy those
wild places, or bring wild things close tous. 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 opportunityfor 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’sa 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 winsthe virus lottery and comes into contact with
one human, and it’s able to make the leapof 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 successfulvirus, and so on, and so on. If someone was going to design the perfect
bioweapon, they might build an aerial vehiclethat you could fill with infection, and then
get to every major city in the world withinhours, where those containers would be opened
up and that infection would be dispersed intounsuspecting 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 connectevery 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 anda disadvantage. We need to be closely connected because we
need scientific information to be passingat 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 worldand 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 evolutionThere’s SARS CoV-3 and SARS Co-V-4 out there
. So yes, we have to change our behavior orthere will be more versions of this spilling
over into humans, causing outbreaks, if notcausing pandemics. Does this give us an opportunity to become
aware of our place in nature in a differentway?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 takea 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 junglesof Central Africa a few miles from an outbreak,
but it applies here, and it applies to ourfuture: “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 hangout 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 canlearn more and all the great other stuff. And if not, just happy that you’re here watching
this video. See you next time.

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