The Biology Behind The Last of Us

[Narrator] This gruesome infection at HBO's The Last of Us

is actually based on this very real parasitic fungus

that turns certain insects into zombies

Growing from her head,

that is the fruiting body that contains the spores.

[Narrator] The creators of the game and the new show

were inspired by zombie carpenter ants, specifically.

They even gave the bugs a quick cameo.

[Dr. de Bekker] So it's a pretty cool Easter egg

that's in the game.

[clicker noises]

[Narrator] The zombies in The Last of Us go through

four stages of infection, which have parallels in nature.

Wired spoke to an expert

on the real fungus reference in The Last of Us.

Those are a lot of spores. [laughs]

[Narrator] To compare those zombies

with similar examples in nature.

Most zombies in TV and film are depicted

as reanimated corpses, but not in The Last of Us.

Viruses can make us ill, but fungi can alter

our very minds.

One thing it does right where a lot of movies do it wrong

is that the infected are still alive

when they're manipulated.

There's lots of examples of this in nature.

[Narrator] The brains of living crickets, cockroaches,

and even mammals can get hijacked by different types

of zombie parasites that alter their behavior.

New research shows that gray wolves get infected

with toxoplasma by interacting

with cougars and their feces, which carry the parasite.

Those that are infected are more risk taking, more bold,

becoming the leader of the pack.

The behaviors that we're seeing are rather are complex.

[Narrator] And in the case of ants,

the Cordyceps parasites spreads throughout the body

as a yeast which secretes chemicals that behave similarly

to neurotransmitters in the brain.

[Dr. de Bekker] It looks like the fungus is avoiding

the nervous tissue.

It's not chewing on it, it's not eating it.

These cells might be close to the brain

and then secretes molecules that manipulate it in such a way

it behaves so precisely to help this fungus spread.

[Narrator] In the first stage, the infected humans

don't show outward signs of their condition.

In the beginning, the ants don't behave differently.

After a few days, you see that the behavior

of the ants definitely is changing.

[Narrator] In the video game, it starts with a twitch.

Twitching, we also see this in our ants.

They seem to have sometimes whole body convulsions.

The fungus is producing a compound that is similar

to Aflatrem.

Certain fungi produce this, and when an animal ingests this

they get staggers syndrome, so they get these tremors.

[Narrator] Then the humans in The Last of Us

get a little hyper, moving at high speed

as if stimulated by chemicals excreted by the fungus.

A similar thing happens

with the zombies in Professor de Bekker's lab.

They're out and about all times

of the day in constant locomotion.

[Narrator] Soon The Last of Us runners lose the ability

to speak.

So do infected ants in a sense.

They don't communicate so well with other

nestmates anymore.

Ants will sniff each other out with their antennae.

The smell of these ants definitely change.

So if an ant starts to smell differently

because she is sick,

they basically will kill her and put her on a discard pile.

[Narrator] There might be a similar social immunity

dynamic at play in The Last of Us.

[Dr. de Bekker] I see a pile of humans infected

by this fungus and sprouting mushroom bodies.

Definitely a pile I wouldn't wanna get near,

you know. [laughs]

[Narrator] Stage two, the stalker phase of The Last of Us

zombies is characterized by obvious fungal growth.

The stalker definitely looks more Cordyceps like.

So all those strings growing off of the head

they very much remind me

of the stalks that we see on our ants, for instance.

It looks like the first bit of a mushroom body coming off

of the left side of her head. That looks pretty gnarly.

[Narrator] Similarly in nature, the parasitic

Masopora fungus radically transforms

its host's physical shape

and gives it a desire to seek out new hosts to infect.

This cicada, you'll see that its missing a part.

It's behind it's genitals, aptly named the butt fungus

'cause it loses its butt.

That's a fungal plug, and the powder are the fungal spores.

This is how the fungus is able to release its spores

and infect other cicada.

In the game and in the show has fungus is able to reproduce

by biting other humans or by contact with spores off of

other humans that have these fruiting bodies

sprouting off of them.

Masopora resembles the fungus in the game a little bit.

[Narrator] Because in The Last of Us and in nature,

zombies have increased energy

and they start to spread out and wander.

Normally, these ants would go forge for food

with their nest mates and nicely follow ant trails.

Instead, they actually just wander around aimlessly.

[Narrator] Could a similar wandering dynamic be at play

with gray wolves infected with a protozoan parasite?

[Dr. de Bekker] Wolves infected with toxoplasma

are more likely to wander away from their former pack

to find a new one and are more risk taking, more bold.

[Narrator] The wolves studied recently in

Yellowstone National Park

could be drifting near to the source of their infection,

cougars, because toxoplasma reproduces

only in the stomach of felines.

In The Last of Us, during the clicker phase,

the fungus has really taken over.

In the clicker, the morphology is changed entirely,

so it makes me think more of the fungus called

chicken of the woods, which is an edible fungus.

[Narrator] At this point, the zombies rely

on echolocation after losing their sight.

In real life, zombie ants might also become impaired

because they start randomly climbing towards light.

[Dr. de Bekker] They will climb up plants at certain

light levels because they don't perceive light so well

anymore and they're trying to search for it

or they're getting themselves off the ground.

This is what we call summiting behavior.

Something that a lot of parasites have evolved to use

as a strategy to transmit better.

The fungus is making the ant find some sort

of microclimate that helps the fungus to develop

its mushroom that releases the spores.

But if you're higher up in the air,

then your spores can be more easily carried by the wind.

[Narrator] And can land on or be eaten by a new host.

Another example of infected hosts acting in the interest

of their parasite are cockroaches that get stung

by jewel wasp, which inject their victims with a kind

of mind control venom along with a brood of eggs.

This changes the behavior of the cockroach entirely,

doesn't wanna fight anymore.

It's this very docile creature that then

follows the wasp back to its borough.

Fully gets eaten alive by this developing egg

that eventually becomes a larva and not a situation that is

beneficial to this cockroach,

but very beneficial to the offspring of the wasp.

[Narrator] Similarly, crickets infected

by the Gordian worm are compelled to act self destructively.

The worm needs to be in water to mate.

So what this worm does is it makes these crickets jump

into water.

This cricket has no business being in the water.

So basically they have committed suicide.

[Narrator] And now the worm can breed

in its natural habitat, the water.

A similar process happens with toxoplasma and mice

which breeds in the digestive system of cats.

[Dr. de Bekker] Rodents that get infected with

toxoplasmosis actually

makes these rodents attracted to cat urine.

It's making it more likely that this rodent will get eaten.

Definitely behavior that is beneficial

to toxoplasma, but absolutely not for the rodent.

[Narrator] In The Last of Us game,

it takes years for the fungus to cover the skin completely

and turn a human into a slow-moving bloater.

But for zombie ants in real life

to succumb completely takes about three weeks

and the beginning of the end starts

with a hyper contraction of the jaw muscles.

Ants bite a branch, can't let go,

and die as they're eaten by the fungus within.

[Dr. de Bekker] When the ant is in place,

the fungus quickly changes its strategy and starts to eat

everything including the brain to produce a fruiting body.

This is when the fungus will change from being a yeast

to forming a mycelium.

Everything inside is just mycelium bursting out of the host.

So this is what we often call the stalk.

You see that on that stalk, there's this brown

bulbous thing attached.

That's the fruiting body, so that carries the spores.

Those are all mushrooms, if you will.

Those are all fruiting bodies.

As the spore gets into the air and as the air moves,

it can easily take it to different places.

So this is why we think that the summiting behavior

is actually helping the fungus to spread its spores.

See the spore cloud, those are a lot of spores. [laughs]

This one would be exuding spores like crazy, looks,

you know, very similar to what we envision the spores

would look like.

They're microscopically small so we can hardly see them

but people that recorded spores coming from mushrooms,

that's kind of what it looks like.

[Narrator] According to a recent World Health Organization

report, fungal infections are estimated to kill

at least 1.6 million people yearly.

Aspergillus Fumigatus can even spread

to the brain of the immunocompromised.

Other fungal pathogens are even showing resistance

in settings like hospitals.

Should we be worried?

Most of the fungi that we know of at least,

they like to grow at a lower temperatures than

our body temperature.

Our bodies are just not great hosts to infect

because the temperature would be too high.

However, in climate change and the Earth's getting warmer,

fungi need to adapt to be able to grow

at higher temperatures as well.

Their optimal temperature will become closer

to our body temperature, making it more likely

that in the future we'll have more fungal infections.

[Narrator] Yikes. But there is one reason

why all of humanity isn't likely to be wiped out

by a mutant zombie fungus apocalypse.

Like in The Last of Us.

In the game and in this show

what you see is that humanity is under brink

of being extinct there and aren't that many humans left.

That's actually not a very smart strategy.

If you wipe out your host entirely,

then eventually there's no hosts to infect

so it will stop to exist as well.

There's so much more going on

that we don't understand quite yet.

We also interact with a lot of microbes

and a lot of organisms that are beneficial to us.

Our microbiome has influence on a lot

of our physiology that leads to certain behaviors.

So who would we be without our parasites?

Who would we beat without our microbes?

I think it's a very interesting philosophical question.