This may shock you, but I’m a massive dork. And I’m weak in the knees for The Last of Us (both the video game and HBO’s adaptation).

So naturally I donned my finest tinfoil hat and decided to write an essay about my theory for why Ellie is immune to the Cordyceps fungus plaguing her world. This normally would have taken many hours of drafting and editing, but using ChatGPT it took only two:

The video game The Last of Us is set in a world where the Cordyceps fungus has evolved to infect humans, transforming them into ravenous monsters known as “The Infected.” In this post-apocalyptic landscape, all hope for a peaceful future seems lost — until a young girl named Ellie is bitten by an Infected and miraculously does not turn. No one in the game knows exactly why Ellie is immune, but pretty much everyone believes she is the key to creating a Cordyceps vaccine that will save the world.

Though Ellie’s immunity is a major plot point in the game, its root cause is the subject of much speculation among fans. A popular fan theory attributes Ellie’s immunity to genetics, suggesting that Ellie’s mother was infected with Cordyceps during the late stages of her pregnancy. This would have given the unborn Ellie in vitro exposure to the fungus. Others have suggested outside of in vitro exposure, Ellie’s genetic makeup may have given her a highly resistant auto-immune system, giving her the necessary boost to fight the fungus and win. While I love the passion and creativity behind these theories, I propose that Ellie’s immunity was actually more environmental than genetic.

HBO’s adaptation of The Last of Us provides the first clue to Ellie’s immunity. In the opening scene, set in an alternate 1960s, a scientist claims that the Cordyceps fungus doesn’t infect humans because their body temperatures are inhospitably warm. He speculates that a warmer climate could trigger Cordyceps to adapt to survive in warmer temperatures. The scene implies that in The Last of Us, global warming caused Cordyceps to evolve to be able to survive inside human hosts.

If we accept the premise that global warming caused the Cordyceps fungus to evolve and infect humans, this raises two new questions:

(1) How warm is too warm for the evolved Cordyceps?

(2) What does Cordyceps do when it’s exposed to inhospitably warm temperatures?

These questions and their answers are crucial for understanding Ellie’s immunity. I propose that:

(1) The evolved Cordyceps can withstand normal human body temperatures, but may not be able to survive in low-grade fever temperatures; and

(2) When Ellie was bitten by an Infected, her body temperature was too warm for the Cordyceps, resulting in a mutation that gave her immunity to the fungus.

To add more color to my responses above: It is well-known that physical activity can increase body temperature. The prequel game The Last of Us: Left Behind depicts the day Ellie was bitten by an Infected. Ellie and her best friend Riley were engaged in rigorous physical activity while attempting to evade the Infected, including running and scaling fences. Ellie is huffing and puffing during these sequences, demonstrating clear physical exertion. This intensive exercise would almost certainly have raised Ellie’s body temperature.

If Ellie had a resting body temperature on the higher end of normal, then it’s plausible that this physical activity would have raised her temperature to low-grade fever territory. This would have elevated her temperature enough to be inhospitable to the Cordyceps fungus when she was bitten.

Assuming that Ellie’s body temperature was higher than normal when she was bitten, the Cordyceps fungus would have needed to mutate to survive within her body. As a result, the relationship the Cordyceps has with Ellie appears to be less parasitic and more in the “uncomfortable roommate” territory.

In other words, Ellie’s immunity may not have much to do with genetics, but instead is a result of the specific circumstances of her infection.

If this theory is correct, it has implications for how communities could combat the Cordyceps infection. Heating stations, like saunas, could be set up to help individuals maintain elevated body temperatures when away from home. Additionally, individuals could be encouraged to engage in physical activity before going outside, to elevate their body temperature enough to potentially make it inhospitable to the fungus.

Under this theory, Ellie emerges as a source of even greater hope than in the game and show. Her immunity to the Cordyceps fungus is not a mere stroke of luck, nor the result of a genetic quirk; It is an indication that the fungus has a weakness that human survivors can exploit rapidly and inexpensively. By replicating Ellie’s elevated body temperature as a defense, communities could potentially protect themselves against the Infected even before developing a more sophisticated prevention program, such as a vaccine or other therapeutics. This new understanding of Ellie’s immunity gives hope that Cordyceps can be beaten — and the odds for humanity might even be good.