URI researcher discusses parasitic fungus at the heart of HBO sci-fi series

KINGSTON, R.I. – March 130, 2023 – If you’re a fan of HBO’s post-apocalyptic series “The Last of Us” – or the video game that inspired it – you’ve probably heard the term Cordyceps.

In the show, the parasitic fungus has mutated, graduating from infecting insects to humans – transforming them into mind-controlled zombies. But it’s not all science fiction.

Cordyceps, formally known as Ophiocordyceps unilateralis, are real – although perhaps unfairly made the villain for entertainment purposes. And there is a real and fascinating story behind them, minus the fantastic storyline.  

Niels-Viggo Hobbs Ph.D. ’16, a University of Rhode Island assistant teaching professor in biological sciences, lends his expertise to carefully walk us through the world of Cordyceps. A life-long fan of science fiction, horror films and “weird fiction” literature, he is a fan of the HBO show – and a researcher of invasive species who also teaches a course on parasitology.

What is Cordyceps? Where is the fungus found? What types of hosts does it infect? And why not humans?

Cordyceps, as it is often commonly called, is a type of parasitic fungus that has a remarkable, unique, and quite disturbing way to infect its host – typically ants.

Cordyceps happens to be a common genus of this parasite, but there are actually hundreds of species of fungus in at least two different taxonomic families, found scattered all over the world, though most commonly in the tropics – all of them having similar general lifestyles as endoparasitoids. This means they infect a host, often some sort of insect, and grow inside of it until they literally burst out – killing the host, often dramatically so.

Funny enough, while these parasites don’t seem to remotely have the ability to infect humans or vertebrates of any kind, that doesn’t mean we don’t often have these fungi inside our bodies. In fact, they’re very commonly willingly ingested in Chinese traditional medicine and there is some evidence to support at least some medicinal value. But, these are dried, and often powdered, forms of the fruiting body – what we’d call the mushroom – not the active, infective organism.

How does Cordyceps take over its host and what effects does it have on the host? How does it multiply?

There is some variation between species, of course, but the basic story is that the fruiting bodies, the mushroom (or ascocarp, as it’s technically called), of the adult organism releases very small spores that are breathed in or otherwise penetrate the prospective host – for example, some insect. Then the spores convert to fungal tissue, which begins to grow inside the insect and spread throughout its body, slowly consuming it from the inside out. But, that’s not even the worst/best part. At some point, the fungus is ready to go to the next phase: reproduction.

Here’s where it gets really interesting. The fungus begins to take over control of the host, basically turning it into a fungus-driven zombie. By the way, this is actually something that many parasites and parasitoids are shockingly very good at – but it’s really dramatic in this organism. The fungus now makes the insect crawl up the nearest tree or tall structure and once the insect-fungus zombie reaches some sufficiently high spot, the insect latches on really well and … the fungus begins to burst forth with its reproductive fruiting body. These burst out right through the body wall of the insect and reach up as tiny but dramatic-looking, finger-like projections. The insect host has already been dead for a while at this point, so there’s no pain involved for the poor thing. Once the fruiting bodies have finished forming, they release clouds of very fine spores, which drift down on everything below, including other would-be insect hosts, completing the life cycle. It’s a very cool and grizzly way of living.

Do we know how Cordyceps affects an insect’s behavior?

The specific metabolic, chemical means by which these fungi hijack their host’s central nervous system is still not fully understood and likely varies a bit across the various species of these parasites, but they appear to release chemicals that trigger basic motor-neuron responses in their hosts – including, in the case of some infected ants, making them latch onto the limb of a high tree with a literal death-grip of a bite, immobilizing the jaws so the host doesn’t fall off the tree. Again, this general ability to hijack a host’s behavior is actually super-common across all the many, many parasite and parasitoid groups of life – including a very simple single-celled parasite, Toxoplasma gondii, which infects rodents and makes them do very dumb things, most notably lose all worry of their top predator and the parasite’s final host and end-goal: house cats!

Is this common behavior for any other type of fungi?

There are many parasitic fungi. In fact, parasitism and parasitoidism (where the host gets killed in order to complete the parasite’s life-cycle) are some of the most common ways for life on this planet to survive. Our own body microbiomes, filled with thousands of bacteria and protist species, include many which are parasitic. Although there are plenty of parasites which can manipulate the behaviors of their hosts, there aren’t too many that so dramatically do it like this.

There are hundreds of species of fungi that fall under the common umbrella of Cordyceps, as a common name, though – so, in this way, there is no shortage of them. And, given their global distribution, they’re really everywhere.

What has “The Last of Us” gotten right and wrong about Cordyceps?

They really get quite a bit right, especially both the host manipulation and the absolute destruction of the host body at the end of the life-cycle of the parasite.

But, there are definitely a couple fundamental aspects they didn’t get right. For one, the fungus doesn’t seem to be able to have some sort of hive-mind communication as depicted in the show. Most notable and unfortunate to me, though, is that they completely skip the dispersal of spores that completes the primary infection of the host. Of course, the only way they could have more appropriately shown this was to have everyone wear very well-sealed masks (which, in fact, may not be enough to protect us), and I don’t think Pedro Pascal would want to be in yet-one-more show where he’s primarily wearing a mask. Interestingly, the original video game that this show is based on does have the non-infected humans wearing masks to protect themselves from infection. Frankly, I wish they’d kept this for the show because it’s both more accurate and also far more terrifying. Instead, they have this weird, gross French-kiss thing that infects folks. Not nearly as cool.

Is the show’s premise of mutation truly farfetched? 
It is a big stretch to have humans being the parasitized, hijacked, and destroyed host organism since Cordyceps don’t at all infect humans. But the general premise isn’t remotely far-fetched. Of course, fortunately, it is science fiction. But, for me, the part that makes it most exciting – and terrifying – is that there is very much a solid basis in reality and biology that we can actually see happen in our remarkably non-fictional natural world.