Human body temperature and brain chemistry may be impenetrable barriers for zombifying fungi.
In the opening minutes of the series, two scientists on a fictional 1968 talk show discuss the microbes that give them pandemic nightmares. One says it’s fungi — not viruses or bacteria — that keep him awake. Especially worrisome, he says, are the fungi that control rather than kill their hosts. He gives the example of fungi that turns ants into living zombies, puppeteering the insects by flooding their brains with hallucinogens.
He goes on to warn that even though human body temperature keeps us fungus-free, that might not be true if the world got a little bit warmer. He predicts that as the thermostat climbs, a fungus that hijacks insects could mutate a gene allowing it to burrow into human brains and take control of our minds. Such a fungus could induce its human puppets to spread the fungus “by any means necessary,” he says. What’s worse, there are no preventatives, treatments or cures, nor any way to make them.
It’s a brief segment, but it had me hooked. It all sounded so chilling and … plausible. After all, fungi like ones that cause nail infections, yeast infections and ringworm already infect people.
So I consulted some experts on fungal infections to find out whether this could actually happen.
I’ve got good news and bad news.
Bad news: Climate change has already helped one fungus mutate to infect humans
I wanted to know if warming has spurred any fungi to mutate and become infectious. So I called Arturo Casadevall. He has been thinking about fungi and heat for a long time. He’s proposed that the need to avoid fungal infections may have provided the evolutionary pressure that drove mammals and birds to evolve warm-bloodedness.
Most fungal species simply can’t reproduce at human body temperature (37° Celsius, or 98.6° Fahrenheit). But as the world warms, “these strains either have to die or adapt,” says Casadevall, a microbiologist who specializes in fungal infections at Johns Hopkins Bloomberg School of Public Health. That raises the possibility that fungi that now infect insects or reptiles could evolve to grow at temperatures closer to human body temperature.
At the same time, humans’ average body temperature has been falling since the 19th century, at least in high-income countries, researchers reported in eLife in 2020. One study from the United Kingdom pegs average body temperature at 36.6° C (97.9° F). And some of us are even cooler.
Fungi’s possible adaptation to higher heat and humans’ cooling body temperature are on a collision course, Casadevall says.
He and colleagues presented evidence of one such crash. Climate change may have allowed a deadly fungus called Candida auris to acclimate to human body temperatures. A version of the fungus that could infect humans independently emerged on three continents from 2012 to 2015. “It’s not like someone took a plane a spread it. These things came out of nowhere simultaneously,” Casadevall says.
Some people argue that the planet hasn’t warmed enough to make fungi a problem, he says. “But you have to think about all the really hot days [that come with climate change]. Every really hot day is a selection event,” in which many fungi will die. But some of those fungi will have mutations that help them handle the heat. Those will survive. Their offspring may be able to survive future even hotter heat waves until human body temperature is no challenge.
Fungi that infect people are usually not picky about their hosts, Casadevall says. They will grow in soil or — if given an opportunity — in people, pets or in other animals. The reason fungi don’t infect people more often is that “the world is much colder than we are, and they have no need of us,” he says.
When people do get infected, the immune system usually keeps the fungi in check. But fungal infections can cause serious illness or be deadly, particularly to people with weakened immune systems
In real life, most human infections arise from breathing in spores. But Casadevall says it’s “not implausible” that people could get infected by eating spores or by being bitten.
Also bad: Fungal genes can adapt to higher heat
I also wondered exactly how a fungus could evolve in response to heat. Asiya Gusa, a fungal researcher at Duke University School of Medicine, has published one possibility.
In 2020, she and colleagues reported in the Proceedings of the National Academy of Sciences on how one fungus mutated at elevated temperature to become harder to fight.
Cryptococcus deneoformans, which already infects humans (though it’s no zombie-maker), became resistant to some antifungal drugs when grown at human body temperature. The resistance was born when mobile bits of DNA called transposons (often called jumping genes) hopped into a few genes needed for the antifungals to work.
In a follow-up study, Gusa and colleagues grew C. deneoformans at either 30° C or 37° C for 800 generations, long enough to detect multiple changes in their DNA. Fungi had no problem growing at the balmy 30° C (86° F), the temperature at which researchers typically grow fungi in the lab. But their growth slowed at the higher temperature, a sign that the fungi were under stress from the heat.
Good news: Human brains may resist zombification
It may not be our body temperature, but our brain chemistry, that protects us from being hijacked by zombifying fungi.
I consulted Charissa de Bekker and Jui-Yu Chou, two researchers who study the Ophiocordyceps fungi that are the model for the TV show’s fungal menace. These fungi infect ants, flooding the insects with a cocktail of chemicals that steer the ants to climb plants. Once in position, the ants chomp down and the chemicals keep the jaw muscles locked in place.
Unlike most fictional zombies, the ants are alive during this process. “A lot of people get the misconception that we work on undead ants,” says de Bekker, a microbiologist at Utrecht University in the Netherlands. She’s glad to see the show “stick to the story of the host being very much alive while its behaviors change.” The fungi even help preserve the ant, keeping it alive even while feeding on it. But eventually the ant dies. Then a mushroom rises from the corpse, showering spores onto the ground where other ants may become infected.