Through a microscope, Matt Nelsen peers at an ant clinging to a branch with a death grip. A long spike shoots from the insect’s back — the calling card of the “zombie” fungus that killed this tiny creature.
This ant now resides among caterpillars, spiders and beetles that all have succumbed to a similar fate, infected by a parasitizing fungus that drained the critters of nutrients, then left them for dead. In some cases, the fungus enveloped the bugs’ tiny brains and started controlling their behavior, hence the zombie nickname.
Now the unlucky insects are part of the Field Museum’s sizable collection of more than 200,000 fungal specimens collected from all continents, sometimes a century or more ago.
To research scientists like Nelsen, each of the specimens tells a fascinating story. But right now, the zombie fungus is stealing the show. Not because it’s a new discovery — scientists have known about the fungus for well over a century. The newfound public interest is instead thanks to HBO’s new hit show, “The Last of Us,” based on a video game of the same name.
In the show, which is rolling out episodes weekly to a hungry audience, zombie fungus has evolved to live in humans. People infected turn into zombies who are controlled by the fungus and driven to spread the sickness. The fungal pandemic obliterates civilization, leaving a post-apocalyptic world that at times may feel a bit too plausible for viewers just emerging to some semblance of post-COVID-19 normalcy.
Is this what’s next for us? Nelsen, a mycologist and evolutionary biologist who is watching the show, doesn’t think so. For starters, the main premise is pure science fiction: Humans are immune from the type of fungus that causes mind control in ants — officially called Ophiocordyceps unilateralis — and it would take extreme adaptations for that to change.
“Making a jump to humans is especially difficult because humans are warmblooded, and that really acts as a defense,” said Nelsen, 46, as he carefully opened tins and envelopes of specimens housed deep inside a part of the museum off limits to the public. “They can’t really survive at temperatures that warm.”
A quick refresh from your high school science class: Fungi are organisms that include everything from yeasts to mushrooms and molds. They often resemble plants, but fungi can’t make food of their own, so they must find a source — like a host to cling to. Some fungi have evolved to develop a mutually beneficial relationship with their hosts. But others, not so much.
In the TV show, the parasitizing fungus spreads through human contact — mainly biting. But in the real world, this is not how the fungi spread among host bugs.
Instead, the fungus attaches itself to the insect’s exterior and burrows inside. For a while, it doesn’t seem to affect its host. But over time, an infected ant begins to behave erratically, wandering around in random directions and eventually convulsing, according to Nelsen.
Then the ant climbs off the ground — the parasite nearly victorious over its zombielike host — and clings to a leaf or branch where it dies. It’s the perfect spot for the fungus to rain down its spores and attract new, unknowing victims.
There are hundreds of Ophiocordyceps — the genus the zombie-ant fungus belongs to. Among those, about 35 species are known to manipulate their host’s behavior. And Nelsen said there are thousands of species of ants not impacted by the fungus.
“So it’s not even all ants they’re going after. It’s a handful [of ants] that they’ve evolved to figure out how to hijack their system and mess with them,” said Nelsen, sporting a T-shirt with a lime green zombie ant he picked up at a mycology conference in Thailand.
Despite the scientific inaccuracies woven into “The Last of Us” storyline, Nelsen is happy to see fungi attracting broader curiosity. As a college student, he took a mycology class on a whim, in part because it sounded “really random and strange,” but he was “blown away by it.”
Part of what amazed him was how understudied fungi are. Despite knowing about 120,000 species (for context: scientists know of about 12,000 bird species), researchers speculate there could be as many as 5 million fungi species yet to be discovered.
“We really know that we don’t know a lot,” he said. “We have a lot of work to do.”
Courtney Kueppers is a digital producer/reporter at WBEZ.
