Could a parasitic fungus evolve to control humans?
The zombie-creating fungus in The Last of Us is real, but there are many other fungi to fear. Of the 5 million fungal species in the world, a few hundred are dangerous to people.
An ant, no longer in control of its body, crawls away from its colony, hangs perilously on a leaf, and waits to die as a fungus consumes its body, emerges from its head, and releases spores into the air.
“They’re like these grim little Christmas ornaments out in the forest,” says Ian Will, a fungal geneticist at the University of Central Florida, where these zombified ants can be found.
What if this parasitic fungus could do the same thing to us?
That’s the premise of the new television show based on the video game The Last of Us in which, as a result of warming temperatures caused by climate change, a fungus takes over the world and turns humans into parasite-controlled zombies.
“In a fantastical way, the logical links are there, but it’s not likely to happen in real life,” says Will. But while scientists aren’t worried about fungi evolving to turn people into zombies, rising temperatures do pose a real risk of making fungal infections worse.
How does the parasite infect ants?
Creator of The Last of Us Neil Druckmann was reportedly inspired by a nature video showing the fungus, Ophiocordyceps unilateralis, infecting a bullet ant. Cordyceps are a broad category of insect parasites, and a popular health supplement. But only ophiocordyceps control their host’s body.
About 35 of these ophiocordyceps fungi are known to turn insects into zombies, but as many as 600 may exist, says João Araújo, an expert on parasitic fungi at the New York Botanical Garden.
The first signs of infection are erratic and abnormal behavior. Scientists think the parasite takes physical control of its host by growing fungal cells around the brain that hijack an insect’s nervous system to control its muscles. It’s unclear exactly how it does this, whether by releasing a chemical or altering a bug’s DNA, says Will.
It’s a process the fungus has been refining within its specific host since before human history.
“Our hypothesis is that they have been coevolving for about 45 million years,” says Araújo.
Are we sure it can’t infect humans?
For the fungus to move to any warm-blooded animal would require some serious evolutionary work.
“If the fungus really wanted to infect mammals it would require millions of years of genetic changes,” Araújo.
Each zombie-creating fungus species evolved to match a specific insect, so unique strains have little effect on an organism except for the one they evolved to infect. For example, a cordyceps that evolved to infect an ant in Thailand can’t infect a different ant species in Florida.
“If a jump from an ant species is hard, to jump to humans—that’s definitely sci-fi,” says Will. “But this idea that temperature plays a role in fungal infections is certainly reasonable.”
A threat from rising temperatures?
Even without a looming threat from parasitic fungi, there are plenty of other fungi to fear.
There are millions of fungal species estimated to exist in the world, and a few hundred are known to be dangerous to humans. One thing that’s protected us from serious fungal infections are our own warm bodies. At around 98°F, human bodies are too hot for most fungal species to spread an infection—they prefer a range of 77°F to 86°F.
(Forget what you think you know about the average human body temperature.)
“One of the reasons why we have skin fungi is they can get between folds of skin. Those are sort of wet, dark places fungi can proliferate that are cooler than body temperature,” says Shmuel Shoham, an infectious diseases expert at Johns Hopkins University School of Medicine.
“As the Earth warms up, there is concern that the change between environment temperature and body temperature won't be as dramatic,” he says. Hypothetically, that would make it easier for fungi that have evolved to withstand hotter outdoor temperatures to also be able to survive inside the human body.
There is one fungal species capable of infecting people that scientists think may have resulted from warming temperatures, called Candida auris.
It wasn’t even known to science until 2007, but in 2011 and 2012, it was suddenly found on three different continents.
“It came out of nowhere,” says Arturo Casadevall, an infectious disease expert at Johns Hopkins Bloomberg School of Public Health. “The idea is that this fungus was out there, and over the years it adapted to higher temperatures until it could break through.”
When they enter the bloodstream, fungi present symptoms similar to a bacterial infection, Shoham notes. For people with healthy immune systems, fighting them off is typically not an issue. But many are not so lucky: The Centers for Disease Control estimates that 30 to 60 percent of patients infected with the fungus have died, although the possibility they had underlying health conditions makes it difficult to determine how pivotal a role Candida auris played.
But when asked if a fungal outbreak akin to COVID-19 was possible, Casadevall says it’s not out of the question.
Considering that possibility, he posits, “Am I worried about an unknown disease emerging and infecting the immunocompetent? Sure.”