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If you’ve been following HBO’s breakout hit The Last of Us—a story of people navigating a post-apocalyptic world where the cordyceps brain infection (CBI) has turned most of mankind into zombies—then you might be wondering if the “zombie fungus” is real (and if it affects humans). While the show is a freaky science fiction series based on a video game, the alarming spore-filled infection seen in the show is actually real.

The Last of Us is reportedly based on the cordyceps fungus (specifically Ophiocordyceps unilateralis), which is a so-called “zombie fungus” that infects ants. But while the fungus is a real thing, experts say it can’t infect humans. Still, it’s understandable to have questions. Here’s the deal.

What is the cordyceps fungus?

Ophiocordyceps unilateralis is a specialized parasite that infects, manipulates, and then kills ants, usually in a tropical setting, according to one scientific paper on the fungus.

“The fungi spreads through spores. A spore lands on an ant and produces a tube that bores through the cuticle into the ant body,” explains Raymond J. St. Leger, Ph.D., a mycologist and professor in the department of entomology at the University of Maryland, who has worked on Ophiocordyceps unilateralis.

“Once in the hemolymph—insect ‘blood’—the fungus multiplies and spreads through the body of the insect,” says entomologist Roberto M. Pereira, Ph.D., a research scientist with the University of Florida.

St. Leger adds that it does so in a “yeast-like form.” The fungus moves on to grow around the ant’s brain, causing the ant to develop “quite specific” behavior during the zombie stage, he says.

How does the cordyceps fungus spread?

There’s a very specific way this fungus makes its way around ants. Once an ant is infected, it “crawls up a plant stem and bites down hard onto a leaf’s vein with its mandibles (jaws),” St. Leger says, adding, “This ‘death grip’ is caused by the fungus colonizing the mandibles.” The fungus also comes out through the legs of the ant as tubes that basically stick the ant to the leaf. “The ant then dies and the fungus produces a fruiting body from the ant’s head—the brain is one of the last parts of the ant’s insides that the fungus digests,” St. Leger says. But, even if the ant doesn’t spread spores over other ants, the spores are often “spread by wind and water, and eventually land on a new healthy host so the cycle can be repeated and new fungal spores can be produced,” Pereira says.

Even creepier? The ant attaches itself to a “pretty specific spot on the underside of a leaf that is near where other ants are foraging or coming down,” says Ian Williams, an entomologist at Orkin. Once the ant dies, the fungus will release spores and, given the ant’s very strategic position, “There’s a high chance those spores will land on others and infect them as well,” Williams says.

Basically, not only does Ophiocordyceps unilateralis turn ants into zombies, it strategically tries to infect as many other ants as possible in the process—an aspect seen in The Last of Us.

“The zombie thing is of particular interest as a creature without a brain—the fungus—is manipulating the behavior of a creature with a brain,” St. Leger says. Consider us creeped out.

What are the signs an ant has Cordyceps?

When the ant is first infected, it’s not something most people would notice. “It might start doing odd behavior that only very specific scientists might notice,” William says.

Ants, however, can pick up on it. An infected ant initially “starts trembling and cleaning itself,” St. Leger says. That’s a heads-up to other ants to get out. “Other ants recognize this behavior as a danger and will carry an infected ant away from the nest and dump it,” St. Leger says.

So, can Cordyceps infect humans?

That’s why you’re here, right? Experts stress that you don’t need to worry about a The Last of Us situation happening anytime soon—from Ophiocordyceps unilateralis, at least. “It’s really specific to a small group of ants,” Williams says. “It’s co-evolved with these ants to have a specific reproductive strategy.”

Of course, some pathogens do jump from animals and bugs to humans, but it seems unlikely to happen in this situation, says Thomas Russo, M.D., chief of infectious diseases at the University at Buffalo in New York. “There are literally billions of microbes on this planet—it’s a microbial world,” he says. “Only a very small part infect humans and, when it comes to fungi, it’s an even smaller number.”

Pereira points out that it’s possible for humans to have an allergic reaction to the fungus, though, noting that one of his employees once developed a severe case of bronchitis while working with a similar insect fungus. “If the quantities of spores are very high in the air, some people may show allergic signs to the fungus, even if no true infection occurs,” he says. However, it’s worth reiterating, you still won't turn into a zombie.

Basically, don’t lose sleep at night over fears that you’ll be turned into a zombie by a creepy fungus that targets ants. But, if you’re a fan of the spook factor, then definitely check out The Last of Us via HBO. New episodes drop every Sunday.

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