One fifth of all mammal species today are bats.

And that's awesome, because they help us out in all sorts of ways.

Like, they pollinate a lot of plants, help regrow forests

and control pests, and their poop is pretty excellent fertilizer.

Plus, they're just really cool.

Some of them can sense magnetic fields, or use sound

to find their food—and, of course, there's the one thing

they all have in common: they can truly fly.

They're the only mammals around capable of powered flight...

without the help of machines.

But they're also somewhat notorious for something else:

being flying sacks of germs.

You might have noticed that part, what with all the talk

about zoonotic diseases that's been happening lately.

Those are diseases that are passed to humans from other animals.

And while bats aren't to blame for everything, they have played

a role in the transmission of at least 11 viruses

—probably 12, counting SARS-CoV-2.

These diseases aren't the bats' fault, of course.

If anything, they're ours.

Research shows that disturbing animal habitats is usually

what causes the transfer of a zoonotic disease to humans.

Still, bats in particular do carry a lot of viruses.

And that's because they have unique immune systems.

Which means we can learn a lot about these pathogens

and their effects by studying bats.

In fact, bat immune systems are so special

that what we learn from them could someday help us treat

a wide variety of conditions, from cancer to diabetes.

And the kicker is: bats probably have weird immune systems

because they fly!

Unlike gliding, flapping flight requires a huge amount of energy.

So, bats have evolved ways to kick their cellular fuel production

into high gear—mainly, by putting their mitochondria into overdrive.

Those are special compartments within cells that turn food into fuel.

But there's a catch!

When mitochondria convert nutrients into energy,

they also create byproducts called reactive oxygen species.

So basically, mitochondrial exhaust fumes,

in the form of really reactive molecules which contain oxygen.

Now, these aren't all bad.

The immune system uses them

to rouse immune cells to action and kill bacterial invaders.

But, they can also cause a lot of damage.

They can weaken cell membranes, mess with proteins,

and even break DNA, and because of that, they play

an important role in diseases like cancer and arthritis.

Cells can try to keep them in check with antioxidants

— compounds that essentially neutralize these overeager molecules.

But those can only do so much, and when the balance gets out of whack,

cells experience a condition called oxidative stress.

This is when most of the DNA damage happens.

So, bats' supercharged mitochondria mean extremely high levels

of oxidative stress—which, in turn, means constantly high levels

of DNA damage.

But since bats don't immediately get super-cancer

after their maiden flight, researchers have long suspected

they've evolved ways to protect themselves

from all this flight-related damage.

And a few years ago, genetics studies found mutations

which boost their ability to detect and repair damaged DNA.

Essentially, they've also turbocharged the mechanisms

that prevent genetically damaged cells from replicating.

Which may also explain why they don't seem to get cancers very often.

So, bats produce tons of energy without damaging their cells.

Sounds pretty awesome, really.

There's just one small problem.

DNA damage can also be a sign of a viral infection,

because viruses need to hijack the cell's genetic machinery to

reproduce, and that process usually involves some strategic snipping.

So, naturally, DNA damage triggers an immune response: inflammation.

Essentially, when cells detect DNA damage or other signs of infection,

they chemically call in white blood cells.

These cells kill and destroy pathogens using a variety

of genetic and chemical tools.

And they also help control how the inflammation unfolds—like,

by bringing in additional white blood cells or switching some of them

from germ-killing to tissue repair once the invasion is over.

This immediate or acute inflammatory response helps to get rid

of the invaders and promotes healing.

But remember, thanks to their supercharged mitochondria,

bat cells experience constant DNA damage.

They can repair this damage thanks to those advanced DNA-repair tools.

But the damaged DNA should still light an immune flare

before it's fixed.

So bats would experience super-inflammation all the time!

And prolonged, chronic, and systemic inflammation isn't so great.

White blood cells and the processes they set in motion

can be really destructive to the body's own tissues.

In short, too much inflammation can lead to organ failure

and even death.

So flight should be a death sentence.

Except, bats have evolved some neat ways

to knock down inflammation, too.

For one thing, they dampen the activity of STING proteins.

These proteins are one of the ways mammalian cells

trigger an inflammatory response when a virus is detected.

Also, a genetic analysis of multiple bat genomes

showed that they're the only mammals that completely lack genes

for PYHIN proteins—another set of inflammation-triggering

sensors activated by damaged DNA.

And those are just part of the story.

It will still be a while before we completely understand

how bats prevent or dampen inflammation in their bodies,

because it seems like every time they look, researchers

keep finding more of these adaptations.

So to recap: we know that to make sustained flight possible,

bats have ramped up fuel production and DNA damage detection

while dialing inflammation down to a 1.

But we know that inflammation is one of the big ways

the immune system fends off intruders.

So, doesn't that leave them open to all kinds of actual pathogens?!

And... The answer is yes!

Around the turn of the 21st century, scientists discovered

that bats act as a reservoir for a lot of viruses

that are extremely dangerous to humans.

This includes filoviruses, which cause hemorrhagic fevers,

like Marburg or Ebola.

Also, henipaviruses like Hendra and Nipah,

both of which can cause fatal brain infections

… and, of course, coronaviruses, like SARS, MERS, and (most likely)

the notorious new coronavirus that started the COVID-19 epidemic.

And there's mounting evidence that bats were involved in

transmitting these diseases to humans, either directly or indirectly,

like by infecting farm animals.

Bats are also suspected of having given us other diseases in the past,

like mumps, measles, and hepatitis B!

But here comes another magic thing about bats:

Even though they're widely infected with notoriously deadly viruses,

they don't actually seem to get sick from them.

The virus can be found in their bodies,

but they don't have any symptoms.

As for how that's possible?

Well, we still have more questions

than answers, but researchers have discovered

a few evolutionary quirks about bat immune systems

that help make that happen.

In part, that's because active viral infections in bats

tend to be pretty short-lived, thanks to their

hypervigilant interferon production systems.

Remember how we said DNA damage is a signal of infection?

That's because viruses try to reprogram cells

to create copies of themselves.

But cells aren't sitting ducks during this process.

In addition to calling out for help—which is that whole

inflammation bit we discussed—cells have

an internal defense mechanism.

They can make a protein called interferon alpha,

which activates genetic and chemical tools that reduce the virus'

ability to multiply and spread.

Every other mammal we know of switches their interferon system on

when an infection occurs.

But genomic studies suggest bat cells always have their

interferon alpha genes activated!

This drastically cuts down the time it takes to react when a virus

is present, so it allows bats to nip the infection in the bud

before it becomes a full-blown disease.

And—of course—there's more.

All mammalian cells contain an enzyme called ribonuclease L which,

when activated, chops up viral RNA to stop a pathogen from spreading.

But, in us and most other mammals, activating this enzyme

takes a complex chain of steps—so it's not super quick.

But bats, on the other hand, can activate it directly with

interferons, drastically speeding up infection containment.

Scientists also think this fast activation of ribonuclease L

helps bats outsmart viruses that have evolved to inhibit

the enzyme before it can be switched on—something HIV

does in humans, for example.

So basically, when bats do get viruses, they're able to quickly

clear them out… for the most part.

They can still fall prey to at least a few, like the rabies virus.

And even quick suppression of a virus can mean it stays around

in a group of bats.

Bats tend to huddle close together when they roost.

Plus, they fly around in the same areas as bats from other colonies,

and they're constantly spraying snot and saliva everywhere

when they echolocate.

So there's a good chance that, during that window when a bat has a

virus that is actively replicating, it will spread it to another bat.

That means even if each bat only hosts a virus for a short time,

it can linger in the population.

And even this probably isn't the full picture.

Mathematical models indicate that, by themselves, bats' social habits

don't completely explain why they host so many viruses.

Instead, research suggests that the viruses themselves have figured out

how to lie dormant and undiscovered—like in the bats' lungs,

spleens, or intestines.

And then, when the bat gets stressed—like when it's roused

from hibernation—that stress temporarily dampens

its anti-virus systems, allowing any hidden viruses to emerge.

This leads to another period of increased

viral replication and shedding.

So again, the bat can transfer the infection to other animals.

Then, its antiviral systems get back up to speed,

and the viruses are eliminated or driven back into hiding.

Still, even when a virus is replicating and being shed,

bats generally don't seem sick—not like a person would

with the same virus.

And that's likely because their inflammation-dampeners

are still running.

So, they're still suppressing a lot of the immune response

that would make them noticeably sick.

And that may also be why viruses that are deadly to us

aren't lethal to them.

It turns out that the most severe symptoms of illnesses

like MERS and Ebola, the symptoms usually responsible

for their lethality aren't caused by what the virus itself

does to the body.

Instead, they're the result of the destructive,

catastrophic inflammation the virus triggers.

This includes an extreme systemic reaction called a cytokine storm,

where an over-release of pro-inflammatory signaling proteins

turns a person's own immune system into their worst enemy.

And some researchers think that our aggravated inflammatory response

may actually be because these viruses have evolved

to dodge the super-refined immune systems of bats.

Basically, they evolved to survive in a host that constantly

and aggressively attacks their ability to replicate.

So when that assault is suddenly weaker in a new host,

they go wild and produce a lot of little virus babies

that send the host's immune system into panic mode.

This kind of overreaction can also happen to bats.

It's just not usually in response to viruses.

Instead, it seems like their unique immune system

may leave them vulnerable to non-viral invaders.

The most infamous example of this is White Nose Syndrome,

a fungal pathogen which has devastated bat populations

in North America.

Some researchers think the bats' dampening of inflammation

—especially during hibernation—makes it easy for the fungus

to infect the bat.

Then, once the bat awakes, it's immune system does react—only,

it goes too far, which can lead to the bat developing a