The Last of Us Finally Reveals Ellie’s True Origin Story to Explain Her Immunity

The season finale of HBO's The Last of Us uses Ashley Johnson's Anna Williams to explain how Ellie became immune to the Cordyceps brain infection.

Anna Williams (Ashley Johnson) stands pregnant, in a yellow dress and green jacket in The Last of Us
Photo: Liane Hentscher/HBO

This Last of Us article contains spoilers.

In The Last of Us, both the HBO series and the video game that it’s adapted on, the Cordyceps brain infection has destroyed the world as we know it. There is no cure for anyone who becomes infected, and they succumb to the mind-controlling fungus within a couple of days.

As far as we know in both the game and the show, Ellie (played by Bella Ramsey in the series) is the only person who has been exposed to Cordyceps and not turned into a flesh-eating monstrosity. While The Last of Us Part I does give some background on Ellie’s immunity, the game doesn’t tell us much about how it actually works or why Ellie is immune when so many others aren’t. However, HBO’s The Last of Us expands on this important part of the game’s lore in the season finale, just as the series has done with other game elements throughout the season.

Episode 9, “Look for the Light,” begins with a tense opening sequence. A pregnant woman named Anna (played by the original Ellie actor from the games, Ashley Johnson) is running from infected while going into labor. She comes across an abandoned farmhouse, where she fights off the infected alone as the baby is being born. Anna realizes that she has been bit in the leg just as she’s given birth, so she cuts the umbilical cord as soon as she can. Even though she cuts the cord immediately after realizing she’s been infected, it turns out that there are still traces of Cordyceps that pass from Anna to the baby, who she names Ellie. However, we don’t know that for sure until Joel (Pedro Pascal) and Ellie make it to the Fireflies in Salt Lake City.

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After Joel and Ellie are attacked, Joel wakes up in a hospital room with Marlene (Merle Dandridge), who explains to him what their doctor’s tests on Ellie’s immunity have uncovered. Marlene tells Joel that the doctor “thinks that the Cordyceps has grown with Ellie since birth” and that it “produces a kind of chemical messenger” that makes “normal Cordyceps think that she is Cordyceps.” She’s immune because, even when Ellie is bitten or otherwise exposed, the Cordyceps that enters her body already thinks that she’s infected. In order to create a cure, Marlene says that the doctor is going to remove the Cordyceps from Ellie’s brain, “multiply the cells in a lab,” and reproduce the chemical messengers to give to everyone.

In The Last of Us Part I, the player can find a recording from the doctor that gives insight into Ellie’s immunity, just as Marlene’s conversation with Joel does in the show. However, in this recording, the doctor says that he is “uncertain” why Ellie is immune. As far as we know in the game, Ellie’s bite at the Boston mall was her first exposure to Cordyceps, and her immunity is presented as more of a miraculous reaction to that than a result of essentially being born with Cordyceps already in her system. In the game, Ellie also has malignant fungal growth in her brain that has to be removed to be studied, but the recording doesn’t mention chemical messengers or how long the fungus has been in her system. Instead, the doctor just talks about how Ellie’s blood tests and MRI’s don’t match what he typically sees in an infected person.

Because we don’t know exactly why Ellie is immune in the game, it seems more likely that there could still be others out there who are immune, or whose immunity could be revealed later on. In the show, however, Ellie’s immunity is caused by such unique timing with regards to when her mom was bitten, when she was born, and when the umbilical cord was cut that it truly positions her as the miracle cure for Cordyceps – making Joel’s decision to save her even more devastating. I may not be an expert in statistics or mycology, but even I can predict that the odds of other children being born under the same or similar circumstances with the exact same result aren’t going to be that high, even with all of the infected running around. Ellie has always been special, but the series makes sure that we know just how rare her immunity is.