The Last of Us | Can a fungus decimate humanity?

Inspired by the video game of the same name, the series The Last of Us has racked up millions of views since its premiere episode premiered on January 15, 2023. But beyond the stunning visuals and acting, what can be said of the science behind the storyline’s starting point: a mysterious mushroom who could decimate humanity?

Chloe Bourquin

special collaboration

A fiction inspired by reality

A deadly fungus that takes control of our brain, makes us aggressive and pushes us to bite our fellow human beings to spread the infection, killing 60% of humanity in the space of a year: this vision of the underworld is not entirely the product of the imagination of the authors of The Last of Us. It is inspired by a very real specimen. “It is a fungus of the genus Cordycepswhich infects ants and modifies their behavior – they are moreover commonly called infected ants “zombie ants”, underlines Simon Dufresne, clinician-researcher in the department of microbiology, infectiology and immunology at the University of Montreal, specialist in medical mycology.

More specifically, the behavior of ants under the influence of this fungus has been extensively studied in the scientific literature. According to a 2011 article published in BMC Ecology, the infected ant “walks around randomly like a zombie”, before biting into a leaf high up to die there. The fungus then grows out of its host’s body, feeding on it, eventually spreading spores and infecting other ants.

This kind of behavior modification is reminiscent of certain diseases that can be transmitted to humans via viruses or parasites, such as rabies or toxoplasmosis. “So if certain microorganisms can change the behavior of humans, why couldn’t fungi one day do that?” says Jennifer Ronholm, professor in the Department of Animal Science and the Department of Food Science and Agricultural Chemistry at McGill University.

Temperature, behavior and transmission

There are three major caveats to this theory. “Firstly, mushrooms do not tolerate high temperatures very well,” the researcher explains straight away. In this case, the majority of them grow at room temperature, around 20°C. “Very few are able to tolerate a temperature as high as 37°C,” adds Simon Dufresne. “So just on that biological basis, there’s an awful lot of species that would never be able to grow inside the human body. »

Moreover, it would be very unlikely that a fungus could take over our brains like Cordyceps controls that of the ant, let alone as described in the scenario of The Last of Us.

“There are tons of brain infections in humans, but none of them can influence behavior in this way. We can have an impact on the neurological level, with hallucinations or paralysis, but this will not cause aggressive behavior, for example, ”says Simon Dufresne.

Finally, as for the transmission of the infection by bite, it is a categorical no for him. “We have co-evolved with fungi for millions of years, but there are very few examples where a fungal infection can be transmitted by contagion from one human to another,” he explains. “There are a few exceptions, for example some species that have colonized our digestive tract like Candida albicans, but only a handful of mushrooms work this way. »

Possible mutation, progression of infection and antifungals

Could a fungus, however, be brought to mutate, in particular because of global warming which could encourage it to adapt to higher temperatures? On this point, the clinician-researcher wants to be reassuring. “I could imagine the displacement of species, ecological niches that expand or shrink,” he explains. “But for a particular species to evolve fast enough to be able to grow at such a high temperature, it would require significant genetic changes. Even if it could adapt to a rise of a degree or two, would that be enough for a species living at 20 or 25°C to suddenly infect humans at 37°C? The gap seems pretty important to me. »

The scenario presented in the series and in the video game also shows that the fungus affects specific parts of the brain depending on the stage of infection. However, “there is no reason to think that a fungus could do that”, emphasizes Simon Dufresne.

Finally, the second episode of the series indicates that no known treatment could be considered to fight against this new specimen. However, this is an unlikely scenario, according to the clinician-researcher. “Most of the antifungals used in humans are broad-spectrum and target fairly universal targets in fungi, such as the cell wall,” he points out. “So they would surely be effective against new fungi. »

The reality of fungal infections

Beyond the fiction, fungi are present everywhere in our environment, and if our immune system is able most of the time to defend itself against them, this is not always the case. Fungal infections kill 1,000 people in Canada and more than 1 million worldwide each year. Among the deadliest are some that attack the brain, such as cryptococcal meningitis. In general, people susceptible to infection are those whose immune system is weakened due to chemotherapy or an advanced stage of AIDS. The World Health Organization (WHO) published in October 2022 a list of 19 fungi that pose a significant risk to public health, pointing out that fungal infections tend to spread faster and faster due to global warming. and increased resistance to antifungals.

“I can’t imagine a scenario like that of The Last of Us can happen, but I invite people to be concerned about the real fungal infections that we actually see in patients, and which require good diagnostic tools and good treatments,” concludes Simon Dufresne.