Meet the Zombie Ant Fungus That Inspired HBO's 'The Last of Us'

If you haven't been hiding under a rock — and at the thought of this particular fungus, who wouldn't be? — you've probably seen or heard of the new HBO hit series "The Last of Us," an apocalyptic nightmare based on the 2013 video game of the same name. The series creepily centers on a group of people left to survive after an Ophiocordyceps fungus evolves to infect humans. You can probably guess the rest, but read on and fear not — we won't be providing any spoilers here.

Zombies are scary for a reason — for many reasons, actually. But one reason is that we humans like the idea that we control our own behavior, which is something zombies ostensibly can't do.

In horror movies, a zombie is just a regular human whose brain gets hijacked by some illness that turns the regular human into a monster. We don't know of any illness that does this in humans to the extent of zombification — although the single-celled parasite Toxoplasma gondii might be more brain-altering than we know — but some other animals on Earth are in danger of getting full-body hijacked by a mushroom.

Meet Ophiocordyceps unilateralis

One example is the fungus Ophiocordyceps unilateralis, which parasitizes carpenter ants in the tropics. Fossil evidence of fungi parasitising insects has been found as far back as 105 million years ago, and over 1,000 fungi with insect hosts are known to exist today, so that's nothing special. But Ophiocordyceps not only takes over the body — it hijacks the ant's behavior during the last days of the ant's life. This phenomenon caught the eye of British naturalist Alfred Russell Wallace when he was in Indonesia back in 1859 — even a newcomer to the area could see that something was seriously wrong with some of the ants in the rainforest. After all, carpenter ants are generally pretty predictable in their behavior: They work as a team and each ant has a very specific job, like building the nest, gathering food or caring for the queen.

What Happens When an Ant Is Infected?

When an ant is infected with Ophiocordyceps, the fungus grows unnoticed in the ant's body for a while, feeding on the ant until the fungal cells account for over half of the ant's body mass. All this time, the ant has been doing its usual jobs and living its life, until one day it begins to act very differently. Breaking off from the colony, it begins to stumble around by itself all day and night, climbing higher and higher into the trees, which is why Ophiocordyceps is called a "summit disease." Eventually, the ant clasps a leaf with its strong mandibles and dies. This chomping of a leaf is how we know this kind of behavior-altering fungal parasitism is pretty ancient — leaf scars created by an ant's death-grip have been found on fossils at least 48 million years old.

Check in a week later, and you'll find the dead ant's body is covered in a brown mat of hair, a beige stalk having erupted from a place near its head. This stalk emits millions of tiny spores, which float away on the air for more ants to step on, thus beginning the whole cycle over again.

Can an Ant Avoid Infection?

Carpenter ants have evolved some strategies for avoiding Ophiocordyceps — at least they try their best. Many ant species groom each other in an attempt to remove spores from their friends, while others spray their nests with fungicidal poisons and seal off parts of their nests when infections arise. A whole ant colony will relocate, if necessary, when Ophiocordyceps comes around.

But how does Ophiocordyceps affect the ant's behavior to the extent that it does? It's a question that's baffled generations of scientists. Over the years, some have argued that the ants aren't hijacked — that certain protocols have evolved in response to a fungal infection — and that the individual ants wander off, not because they are being influenced by the fungus, but to avoid infecting the rest of their colony.

Does the Fungus Preserve the Brain for a Reason?

A 2017 study published in the Proceedings of the National Academy of Sciences found that it's possible the ant's brain isn't involved in the whole process at all. The paper found that as the fungus grows inside the ant's body, its cells create an interconnected network of fibers that almost entirely fill the body cavity of the host ant — and it is that network that is most likely coordinating the ant's movements. Using AI they discovered it's possible the fungus didn't need to enter the ant's brain at all in order to manipulate the insect's head, thorax, abdomen and legs.

"We found that a high percentage of the cells in a host were fungal cells," said David Hughes, lead author of the study and associate professor of entomology and biology at Penn State, in a press release. "In essence, these manipulated animals were a fungus in ants' clothing."

Pretty much everything but the brain is entirely taken over by fungal cells. Which leads the researchers to believe that the fungus might be preserving the brain for a reason — perhaps in order to help the ant to survive until it can perform its final "death bite."

Another 2017 paper co-authored by Hughes an published in the journal Plos One, explored genes that seem to allow Ophiocordyceps to activate different genes during the day and night to control its ant host.

Although these findings are compelling, it's still not crystal clear what's going on between Ophiocordyceps and its victims. What is clear is that it would be extremely difficult for a fungus to start hijacking human bodies in the way Ophiocordyceps has its way with ants. At least we have this small comfort.