'The Last Of Us' made us wonder: Could a deadly fungus really cause a pandemic?

From a scientific perspective, the new HBO show The Last Of Us is telling us a whopper–a mesmerizing whopper.

In the show, climate change has fueled the rise of a new pathogen, which sweeps around the globe infecting humans, turning them into zombies and controlling their brains.

The cause of the infection, however, is a bit surprising. "Not bacteria? Not viruses?" a TV journalist asks a scientist in front of a live audience during the first episode.

"Fungus," the scientist answers. The audience laughs.

"Yes, that's the usual response," the scientist says.

And then he goes on to explain why fungi are a dire threat to humanity, pointing to the idea that rising temperatures on Earth will drive them to be more infectious to humans.

When I watched this scene, I have to admit that I chuckled out loud, too. "A fungus wiping out humanity? Come on." I thought. "That is not even a remote possibility."

But then, for this story, I started to research the possibility of a fungus triggering a pandemic, and I have to admit: I was wrong. I shouldn't have chuckled – not even a little bit. Because there's growing evidence – real evidence – that climate change may in fact make this class of pathogens more dangerous to humans.

Viruses v. Fungi? What's the bigger danger to humanity?

For the past decade, I've reported on infectious diseases. Often I've asked scientists, "What keeps you up at night? What types of pathogens could cause a horrible pandemic?" Every time, I've heard one type of pathogen, over and over again: viruses. The specific family of virus scientists worry about varies, but it's usually influenza, a SARS-like coronavirus or a paramyxovirus (which cause horrible illnesses such as Nipah and Hendra).

No one has ever said "fungi" in response to my query. And a fungus has never caused a massive pandemic, similar to what the world is experiencing with COVID-19. The reason? Viruses have several big advantages over fungi when it comes to infecting people. For starters, they spread much much faster.

"So the big advantage, if you will, for viruses is that one viral particle can become thousands of particles in a very short period of time," says Dr. Aileen Maria Marty, who's an infectious disease specialist at Florida International University and has worked with the World Health Organization on several recent outbreaks, including Ebola in West Africa in 2014 and Zika in 2016.

"Furthermore," Marty says, "as [the virus] produces more viral particles, it has the propensity to have mutations."

That inclination to mutate means viruses can change and evolve much faster than fungi. "Those mutations can lead to a new version that could be more dangerous quickly," Marty says. So all of a sudden--say, in a few weeks--the virus can start evading people's immune systems. Then in a flash, the whole world becomes susceptible to the virus again, as we've experienced with omicron and its myriad variants.

Here's the key part: Fungi generally can't do this, studies have shown. They mutate more than 10,000 times slower than viruses, on average, scientists have estimated. (The exact estimate depends on the specific virus and the fungus.)

Furthermore, Marty emphasizes, people who have healthy immune systems can fight off fungal infections before they become dangerous. "The reality is that most immuno-competent people do not get sick from a fungus entering their body." The same cannot be said for many viruses.

So I ask Marty: "If you had to put money on what's going to cause the next pandemic, would you put it on a fungus or virus?"

"I would put it on a virus. I really would," she says without hesitation. "But that doesn't mean that we shouldn't pay attention to fungi because many, many, many, many people die every year from fungal infections." In fact, more than 1.6 million people die from fungal infections each year. And there's a growing concern that these deaths will rise, in part, because of climate change.

Fungi are changing--and could become more infectious to people

So here's where the show The Last of Us gets the science right – or at least partially right. Most fungi live out in Earth's environment, such as in the soil and on plants. They can't survive in people because humans are too hot, says Laura Goodman, who studies pathogen genomics at Cornell University.

But as the scientist in the first episode of The Last of Us explains: "Currently there are no reasons for fungi to evolve to be able to withstand higher temperatures. But what if that were to change? What if, for instance, the world were to get slightly warmer? ... Well, now there is [a] reason for fungi to evolve."

In other words, could climate change pressure fungi to survive at higher temperatures, like that of the human body, making them more capable of infecting humans?

There's some evidence that this process is already happening, at least with one fungus species, called Candida auris. It's an emerging species, first detected in a 70-year-old woman's ear in Japan in 2009, and it's already emerged independently on three continents.

"This fungus species is pretty nasty," Goodman says, "because it is resistant to many of the drugs that we have available.

"And not only that, it also seems to have a strong advantage in changing in such a way that it can cause disease in people."

Studies suggest C. auris can mutate as quickly as some viruses, and that rising temperatures in some parts of the world may have pushed it to survive at higher temperatures.That may have helped it gain the ability to infect people.

Right now, C. auris is a major problem in many hospitals, including those in the U.S., Goodman says. But it's mainly a risk for people with compromised immunized systems and who are very sick. "For many people, it's probably harmless."

She is concerned that could change.

"Fungal infections definitely keep me up at night," she says, "because I see all the work that is performed on bacteria and viruses and how much we know about all these microorganisms. And then we look over at the fungal pathogens, and I see we know so much less."

And, even though no fungi has caused a deadly pandemic in people, they have caused horrific outbreaks in wildlife.

"All you have to do is look at bats with white nose syndrome or frogs and salamanders with chytrid fungus," she says. "These fungi are really devastating pathogens for these species, capable of essentially wiping out entire groups of these wildlife."

So, it turns out, the writers of the HBO show got more of the science right than I expected. But rest assured, there is no current evidence of fungi out there on the horizon that will infect our brains and control our minds.

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