Editor's Note: The following contains spoilers from Episode 9 of The Last of Us.As expected of any good season finale, Episode 9 of The Last of Us Season 1 was packed with answers to many of our most pressing questions. Over the course of its 45 or so minutes, we learned just how attached to Ellie (Bella Ramsey) Joel (Pedro Pascal) has grown and how far he is willing to go to protect his relationship with her. We also learned what exactly is the Fireflies’ plan to save humanity from Cordyceps and how they will serve as foes to Joel and Ellie in Season 2. But, most importantly, we learned what it is that makes Ellie capable of coexisting with the Cordyceps colony in her body.

What makes Ellie immune has been an age-old question for Last of Us fans, including those who played the original game. The internet is full of theories, most of which include her parentage. Now, the HBO version of the story has finally settled the matter of her immunity. But what is it exactly that makes Ellie so special? And what does it mean for her, Joel, and the future of the show?

‘The Last of Us’ Season 1 Finale Gives Us Insight on Ellie’s Past

Ashley Johnson as Anna in The Last of Us Season 1 Episode 9
Image via HBO

The first few scenes of “Look for the Light” take place not in our Cordyceps-ridden present, but in the Cordyceps-ridden past. The infection is already going strong when we are introduced to Anna (Ashley Johnson), a pregnant woman looking for a safe place to give birth to her baby. After a long time running through the woods, she finds shelter inside an abandoned house. Sitting on the floor of an empty room, she times her contractions and pushes hard to help her child out into the world.

But just as her baby is taking her first few breaths, Anna is attacked by an infected. Though she manages to kill the creature with a certain ease, she isn’t quick enough to avoid a bite to her right leg. Level-headed, but still desperate to save her child from the same tragic fate as hers, Anna cuts the umbilical cord as fast as she can so that the infection doesn’t have time to travel through it to the baby’s body. Despite the child’s desperate cries, Anna also refuses to nurse her so that she doesn’t get infected.

Merle Dandridge as Marlene on the last of us episode 9
Image via HBO

Little Ellie is still crying when Marlene (Merle Dandridge) and a Firefly soldier reach the house in which Anna is hiding, cradling her baby against her chest and holding a knife to her own neck. Anna lies that she cut the umbilical cord before she was bitten and asks Marlene to take care of Ellie for her. She also pleads with her long-time friend and fellow Firefly fighter to kill her before the Cordyceps take control of her brain. Initially reluctant, Marlene picks up Ellie and gives her to the other soldier before taking out a gun and shooting Anna on the head.

The rest, as we know, is Season 1 history: Ellie was raised in a FEDRA facility, and trained from a young age to become a soldier. Eventually, she was bitten by an infected during a nightly escapade. While most people fall under full control of the fungus in less than two days, Ellie remains asymptomatic long after the infection, which suggests that there is something inside her that makes her immune to Cordyceps. In order to find out what exactly it is that protects her from the infection and possibly create a vaccine, Marlene tasks Joel and Tess (Anna Torv) with taking a now 14-year-old Ellie to a Firefly research facility in Salt Lake City.

RELATED: New 'The Last of Us' Featurette Explains Joel's Season 1 Finale Choices

Ellie Grew Up With the Cordyceps in Her System, and This Isn’t Exactly Good News

Bella Ramsey as Ellie in the Season 1 finale of The Last of Us
Image via HBO

It is only upon reaching said research facility - a deactivated hospital - that Joel learns the truth about how the Fireflies intend to use Ellie to create a vaccine. After being captured by the Fireflies, Joel wakes up in a hospital room with Marlene by his side. Ellie is nowhere to be found. As Marlene explains to him, she has been taken to surgery.

Wait, surgery? Yeah, Joel doesn’t like the sound of that either. Marlene, however, is quick to assure him that this is absolutely necessary. You see, the leading Firefly theory as to why Ellie is immune to Cordyceps is that the fungal spores inside her body grew up with her. Somehow, probably through that umbilical cord that Anna lied about cutting off before getting bitten, she was exposed to the fungus when she was still a baby. Thus, when she was bitten by an infected during her trip to the mall with Riley, in Episode 7, the spores that entered her system took her for one of them. This, in turn, stopped the infection from developing.

Considering just how gruesome the Cordyceps infection is, this certainly sounds like great news for Ellie, and even greater news for humanity as a whole. After all, if there is a fungal charge to which people can be exposed without danger or an age in which the infection isn’t able to take hold, this means that there are great chances for a vaccine on the horizon. Since antifungal vaccines in general are still out of humanity’s reach, isolating the spores that make Ellie immune and using them to curb the evolution of the disease would be one hell of a scientific breakthrough.

However, creating a vaccine is never simple, and the Cordyceps shot has its fair share of extra complications. The problem lies in the “isolating the contaminant agent” part of the equation. As Joel is quick to point out, the Cordyceps fungus attaches itself to the human brain. Thus, in order to take the spores out of Ellie’s body, the surgeon would have to cut into her brain, killing her in the process.

Are There Any Other Immune Survivors in ‘The Last of Us’?

Pedro Pascal and bella ramsey as joel and ellie in the last of us episode 9
Image via HBO

Joel is immediately angered by the realization of what the Fireflies intend to do to Ellie. He tries to argue with Marlene that she should let Ellie go free, but she isn’t sympathetic to his pleas. In the end, as usual, all that is left for him is violence: in order to save his surrogate daughter, Joel steals a gun from a Firefly soldier and takes out almost everyone in the research facility, Marlene and the surgeon included. He rescues a still unconscious Ellie and drives her the closest he can get to Wyoming, where his brother Tommy (Gabriel Luna) lives in an independent commune. When Ellie wakes up, she asks Joel about what happened at the hospital, and he lies to her, claiming that there are many others like her and that the Fireflies had been running tests on a bunch of immune survivors. Sadly, however, the hospital was attacked by raiders just after they got there, and it was only a matter of luck that he and Ellie managed to escape.

Ellie doesn’t sound too convinced by Joel’s story, and neither should she be. For starters, this whole raider attack thing sounds way too convenient. Secondly, and perhaps most importantly, if there are indeed dozens of immune survivors, why on Earth was Marlene so intent on getting Ellie in particular to Salt Lake City? The math doesn’t add up, and that’s because, as far as viewers and characters are aware, Ellie is indeed the only person that has managed to live a normal life with Cordyceps in her brain. So far, there are no other immune survivors in the world of The Last of Us. Joel may have run away with humanity’s only shot at a vaccine. Whether he was right or wrong to do so, well… That is up for the viewers to decide.

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