'The Last of Us': How is Ellie Immune to the Cordyceps Infection?

In The Last of Us, the world is ravaged by the mutated Cordyceps fungus, which takes over its hosts at varying speeds and ferocity.

The brain infection turns humans into mindless creatures—similar to zombies—though it controls its target while they are still alive before eventually killing them; the disease is transferred from host to host through biting.

Humans can turn into three types of Infected—Runners, Clickers, and Bloaters, each more dangerous than the last. While humanity has been decimated by this brain infection for 20 years, there is one exception: Ellie Williams (Bella Ramsey).

Warning: This article contains some spoilers for The Last of Us game, and thus will contain storylines likely to appear in the HBO adaptation.

How Is Ellie Immune to the Cordyceps Infection in The Last of Us?

In the opening episode of The Last of Us, Joel Miller (Pedro Pascal) and his partner Tess (Anna Torv) are tasked with a dangerous mission by Fireflies leader Marlene (Merle Dandridge): take Ellie across the U.S. and bring her to one of their labs.

The Fireflies are a rebel group trying to fight against military control, and the reason that Ellie has to be taken transferred to their lab is that she is immune to the Cordyceps infection, which she discovered when she was bitten by an Infected but did not turn.

Ellie, therefore, could be the key to saving humanity as her blood may provide an answer to creating a cure for the virus, and it is up to Joel to bring her safely to the location of a Firefly lab where she can help scientists work on a vaccine.

Newsweek won't elaborate on exactly what happens during their journey, or the game's climax, with regards to the aforementioned mission so as not to ruin the surprise for HBO viewers who have not played the game.

What viewers may be wondering is exactly how Ellie can be immune to the Cordyceps infection when so many are not. Well, the characters in The Last of Us grapple with the same question.

Neil Druckmann, the game's creator and co-president of developer Naughty Dog, has never confirmed how Ellie is immune, though the 'why' is obvious: If Ellie was not immune, there would be no story because the game revolves around Joel and Ellie's journey to get her to scientists seeking a cure.

However, the game provides a hint regarding Ellie's immunity in the game's epilogue where players can find a 'surgeon's recorder'. The artifact explains that Ellie's condition is an anomaly that scientists have not seen before in any of their previous subjects.

The Fireflies have been experimenting on other Infected, but none has shown the same ability to fight off the virus as Ellie. It is explained that despite the Cordyceps fungal infection being present in her system, she remains unaffected. This is described as being akin to the "discovery of penicillin," so it appears that Ellie's immunity is simply a result of a mutation.

One fan theory suggests that Ellie's immunity could be linked to her mother, who died when she was bitten by an Infected, potentially whilst still pregnant with Ellie.

Druckmann, who is also a co-creator of the HBO show with Craig Mazin, told Newsweek how he had written a storyline about Ellie's mother which he was never able to make into a game but will be used in the HBO show.

"There was a story that I had written after the game came out—about Ellie's mom, that was going to be realized in this one way, and then it just kind of fell apart so it was always just sitting on a shelf," he said.

"Craig's excavation of what The Last of Us was, and just talking about everything that we had developed within Naughty Dog and the history of the world, and all these different factions and these relationships, I mentioned this story to him and his eyes went wide and he was like, 'Oh, my God that has to go into the show.'"

Therefore, this plotline may well provide a definitive answer as to why Ellie is immune to the Cordyceps infection.

The Last of Us airs on Sundays at 9 p.m. ET on HBO and HBO Max.