Warning! This article contains spoilers for The Last of Us episode 9.The Last of Us episode 9 reveals the truth about Ellie's immunity to the Cordyceps brain infection that has ravaged mankind. Since The Last of Us episode 1, the core plot point of the series has been about Joel taking Ellie to the Fireflies due to her immunity from the infection that ended the world. If Ellie can reach the Fireflies, the freedom-fighting militia is adamant that they can synthesize a cure for the Cordyceps infection, meaning Ellie can help to bring the world back from the brink of destruction.
While Ellie's immunity has always been prevalent throughout The Last of Us, the show has failed to adequately explain exactly how her immunity is possible. With Ellie's bite mark - which she received in The Last of Us episode 7's tragic ending - showing signs that the Cordyceps fungi are indeed in her system, question marks have always been raised over why Ellie does not get infected like the rest of humanity. However, these questions are finally answered with The Last of Us episode 9 and the culmination of season 1.
Ellie Is Immune In Last Of Us Because Anna Was Bitten During Childbirth
The Last of Us episode 9 opens with an original scene for HBO's adaptation in which Anna, Ellie's mother, is shown fleeing the infected in the midst of labor. After running to an abandoned house in the middle of nowhere - which she expected to be filled with Fireflies - Anna locks herself in a room upstairs. However, an infected tracks her and smashes through the door causing Anna - played by original Ellie actor Ashley Johnson after Joel's game actor Troy Baker appeared in The Last of Us episode 8 - to have to fight it off.
The ensuing struggle sees Anna kill the infected, not realizing that the stress of the encounter caused her to birth her child which she names Ellie. Anna also realizes that she was bitten during the fight, before swiftly cutting the umbilical cord connecting her and Ellie. This is the cause of Ellie's Last of Us immunity. Given that the umbilical cord provides babies with oxygen and nutrients, the Cordyceps present in Anna's system is transferred into Ellie. However, Ellie was not infected, as explained by Marlene later in the episode.
Ellie’s Immunity Comes From Cordyceps Already In Her System
Midway through The Last of Us episode 9, Marlene relays the true reason Ellie is immune to Joel which the Fireflies' surgeon has discovered. According to the surgeon, The Last of Us' fungi Cordyceps which was present in Ellie's system from her birth acts as a "chemical messenger" of some kind. This messenger acts somewhat like a barrier. Whenever Cordyceps has entered Ellie's system - such as the bites shown in The Last of Us episodes 2 and 7 - the Cordyceps already in her system make the new fungi entering her body think she is already infected, rendering her immune from its effects.
Could A Cure Have Been Made In The Last Of Us Episode 9?
The revelation of Ellie's immunity in The Last of Us episode 9 begs the question of whether a cure could actually have been made. Marlene states that the surgeon intended to remove the Cordyceps that has been with Ellie since birth, and multiply the cells - therefore producing more of these "chemical messengers". Marlene tells Joel that the surgeon thinks this could create a cure, after injecting the chemical messengers Ellie's body produced into the wider population.
Ultimately, Joel's actions in Last of Us season 1, episode 9 make it irrelevant whether this would have led to the Fireflies creating a cure, as the only surgeon who possibly could have is dead. However, there are some hints throughout the episode that provide evidence both for and against a cure being possible. Firstly, Marlene's wording is interesting. In The Last of Us episode 9, she states to Joel: "He thinks it could be a cure, Joel." The use of the word "thinks" particularly is interesting, as it means the surgeon is not 100% sure a cure is possible, only that he hopes to produce one from Ellie's condition.
However, Joel also lies at the end of The Last of Us episode 9 that other immune people have been cropping up across the country. The fact that Ellie is seemingly the only person immune to Cordyceps in history means that, whether the surgeon is sure or not, he will still be dedicated to trying to craft a cure. The reality is, both the original The Last of Us and HBO's TV adaptation of the game remain ambiguous on whether a cure would be possible. Instead, the emotional crux of The Last of Us episode 9 falls on Joel's decision and the completely understandable yet controversial nature of it.