Why the scary fungus in 'The Last of Us' won't cause an apocalyptic outbreak

Question: Did you inhale any fungal spores today?

Answer: Yes. Absolutely. Tons of them. They're everywhere.

"Anyone that's been outside has inhaled fungal spores," said Vincent Bruno, a scientist at the Institute for Genome Sciences at the University of Maryland School of Medicine who researches fungal diseases. Fortunately, however, your mind wasn't infected with the fungus Cordyceps, which in the HBO series The Last of Us drives a global outbreak that transforms the hapless infected into vicious, zombie-like characters, hellbent on spreading disease to the dwindling (and often desperately starving) human populace.

Such a fungal infection in humans is captivating fiction. Yet in reality, Cordyceps does regularly hijack the minds of a bullet ant species. The fungus cleverly compels the ants to climb to favorable environments for the fungus to grow. "It turns them into zombies," Theresa Gildner, a biological anthropologist at Washington University in St. Louis who researches parasitic disease, told Mashable. Cordyceps then boldly emerges from the ant's head, allowing the spores to spread to other unwitting ant hosts. Indeed, nature can be savage. This very fungal savagery in ants inspired The Last of Us video game that led to the popular new series starring Bella Ramsey and Pedro Pascal.

Crucially, although some fungal spores can cause serious human infections under certain conditions, a fungal parasite that controls our minds is, thankfully, not in the cards. Here's why.

Why the Cordyceps fungus won't infect people

When you stroll through the woods, it's the fungus' world; we're just passing through it. The organisms feed on dead critters, recycling bodies and organic matter into the soil, while forming vast underground networks. They thrive in the natural world. There's nothing to stop them.

"Out in the world, there's no immune system trying to kill them," Bruno told Mashable.

But herein lies a problem for most fungi. They can rarely survive in human bodies. "Humans are really, really good at staving off fungal infections," Bruno underscored. There are three potent reasons why:

• Advanced immune system: Our immune systems are much more complex than an ant's — and a fungus can't overcome our multi-layered defenses. "Our immune system recruits white blood cells to come and kill it," Bruno emphasized.

• We are endotherms: This means we're warm-blooded. Many fungi can't survive in such hot environments.

• Primed immune system: Inside the human gut, microbes flourish, including certain fungi. This allows our immune systems to recognize and differentiate between normal fungi and fungal invaders we come across in nature. Our immune system is ready to attack foreign, potentially harmful, spores.

  A shot of a fungal-infected character on "The Last of Us." Credit: HBO

How some fungi cause serious human disease

Humans, however, aren't completely out of the fungal woods. Certain fungi can seriously infect us when we're enfeebled.

"To experience a fungal infection, there has to be something wrong with your immune system," explained Dr. Luis Ostrosky, an M.D., fungal infection expert, and professor of infectious diseases at UTHealth Houston. He is also chief of epidemiology at the academic hospital Memorial Hermann.

Compared to 30 years ago, these types of fungal infections are much more common, he noted. And it's because modern medicine necessarily allows for more and different types of immunocompromised patients. These include people who've had organ transplants or other major, life-saving surgeries that weren't possible in the past, as well as those undergoing cancer treatments. "We're creating a paradigm shift in the amount of people who are immunocompromised," Dr. Ostrosky said. Importantly, infectious disease doctors know how to identify and treat such diseases.

In hospitals, one of the most frequently seen fungal infections, with some 25,000 cases in the U.S. annually, is Candida. Candida is everywhere, and our immune systems keep it in check. But for those with weakened immune systems, Candida can cause a serious blood infection. Worse yet, a strain called Candida auris has emerged in recent years. It can be resistant to some, or all, antifungal drugs, and has caused outbreaks in hospitals, Dr. Ostrosky noted. "Candida auris is an emerging fungus that presents a serious global health threat," says the CDC. Then there's Aspergillus — a mold we all breathe every day. But for those undergoing chemotherapy or taking immunosuppressants, the common fungus can be serious. "We used to see it once a month," explained Dr. Ostrosky. "Now we see it once a day. It's one of the most deadly fungal infections."

Harmful fungal spores can easily spread outside the hospital, too. Valley fever is caused by the fungus Coccidioides, which thrives in warm and dry soils, like in the Southwest. People breathe it in, and some folks are vulnerable. Valley fever is now appearing in new places, like Washington state and Chicago. This could be the product of our relentlessly warming climate driving new places for Coccidioides to thrive, Dr. Ostrosky said.

This might also be a potential harbinger of future, potentially unsettling Valley fever scenarios in dried or drought-stricken regions. "Imagine wind-driven spores around the country," Dr. Ostrosky mused. "That's a more likely scenario than fungus-driven zombies."

In nature, mind-controlling parasites are very real

Although the human infection in the The Last of Us isn't real, in the greater animal kingdom and beyond, parasites regularly alter the minds of their unwitting hosts. For them, a mind-controlling scourge isn't tantalizing fiction.

"There are so many terrifying, but interesting, examples," said Gildner, the biological anthropologist.

• Worm-infected ants infect grazers: A parasitic worm (Dicrocoelium dendriticum) affects the brain of ants, driving them to climb to the top of a blade of grass at night rather than returning to their colony, Gildner explained. This boosts the odds that a grazing animal — that the worms seek to infect — will eat the ants. "This is the parasite's ultimate goal, to get inside a grazing animal so it can complete its life cycle and reproduce, and is just one example of how many parasite species manipulate their hosts to achieve a very specific aim," Gildner said.

• Brain-infected crickets: Extremely thin horsehair worms infect crickets and other insects. For example, in caves, such as those in Carlsbad Caverns National Park, crickets taking a sip of water imbibe the young worms. The worms grow inside the cricket, consuming the cricket's body and ultimately making contact with the brain. They likely release mind-altering chemicals into the cricket's brain. Ultimately, the crickets, who can't swim, are compelled to leap into the water, allowing the fully grown worm to emerge...and reproduce.

• Devious parasite in cats: The parasitic microbe Toxoplasma gondii has adopted a clever way of infecting cats. It reproduces in cats, then sheds in cat feces. Rodents eat the poop, and invariably the microbe, too. Once inside, the tiny microbe will travel to the rodent's brain, forming cysts in a part of their mind associated with fear. Research shows this causes decreased fear and alarm in rats, allowing cats to more easily hunt and eat them. And the cycle continues.

Parasitic mind control, folks, is real. But rest assured you won't be chased down by a Cordyceps-infected zombie. At least, as far as anyone knows.