Warning! This article contains spoilers for both HBO's The Last of Us episode 1 and the original game.HBO's The Last of Us episode 1 revealed a significant element about Ellie's character, and here is everything you need to know about the reveal in question. The Last of Us centers on two different places in the world's timeline. The show begins with a prelude to the infection that will decimate humanity, with two scientists giving their views on humanity's greatest threat, to which one says fungal infections could be more severe than bacteria or viruses. The show then flashes to 2003, with Joel, Tommy, and Sarah witnessing the outbreak of the Cordyceps infection firsthand. The show then jumps 20 years to 2023 after humanity is all but gone.

The Last of Us shows the post-apocalyptic life of the Boston quarantine zone through the eyes of Joel (Pedro Pascal) and Tess (Anna Torv), who are introduced to Ellie (Bella Ramsey) - a girl they are tasked with smuggling out of the QZ for the Fireflies for unspecified reasons. Ellie is held in high regard by Marlene, leading Joel and Tess to agree to escort Ellie in exchange for a truck they can use to search for Joel's brother Tommy. However, the end sequence of The Last of Us episode 1 establishes why Ellie is so important to the Fireflies, and why they want her kept alive at all costs.

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Ellie's Immunity Explained

That reveal is that Ellie has something of an immunity to the Cordyceps disease. At the end of The Last of Us episode 1, Joel, Tess, and Ellie are captured by a FEDRA agent outside the Boston quarantine zone who scans them for infection, an important scene taken from The Last of Us game. Upon scanning Ellie, she turns and stabs the agent in his leg, causing Joel to have to kill him. Tess then finds the scanner, which reads that Ellie is infected. After a brief moment of panic, Ellie reveals the bite on her arm and that she received it over three weeks ago. Most infected in the world of The Last of Us begin to turn within 24-48 hours of being bitten, which proves Ellie's immunity.

However, when showing her arm, the bite's infection is seen running through Ellie's forearm, which means the Cordyceps fungi are still present in her system. However, given how she has been "infected" for over three weeks, it means that the infection does not affect her in the same way as other humans in HBO's The Last of Us. The main symptoms of the Cordyceps infection are that it causes people to lose their higher brain functions, becoming volatile and driven only to spread the infection. Ellie, however, has some kind of immunity to this, with the infection not harming her as it does the rest of humanity.

Why Ellie Is Important To The Fireflies

This explains why Ellie is so important to the Fireflies. Given how the world has been in disarray for over 20 years, and humanity is all but wiped out with any surviving pockets confined to QZs like Boston, the Fireflies see Ellie as a potential way to revert humanity back to the way it was. Through Ellie's immunity, the Fireflies hope to synthesize a cure for the Cordyceps brain infection. This explains why Marlene, leader of the Boston Fireflies and a Last of Us character taken from the game, was so careful with Ellie by making sure she did not turn for a few weeks after being bitten.

Marlene and her team were then prepared to abandon their fight against FEDRA in the Boston area and take Ellie to the Massachusetts State House. There, the team of Fireflies they were to meet would likely then take Ellie to their base to run tests on her and uncover the cause of her immunity. One thing that The Last of Us episode 1 establishes, through Tess and Joel's surprise at Ellie's immunity, is that it is certainly uncommon. Having someone as rare as Ellie in their hands would naturally lead the Fireflies to seek a cure to avoid The Last of Us' infected and clickers from becoming even more commonplace.

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How Did Ellie Become "Immune"?

As of the end of The Last of Us episode 1, it is unclear how Ellie became immune to the Cordyceps disease. However, there are some theories that have spawned from the game of the same name, on which The Last of Us is based. One of these theories ties to the inclusion of Ashley Johnson in HBO's The Last of Us. Johnson was the original voice actor and mocap performer for Ellie in both The Last of Us Part I and The Last of Us Part II and was confirmed to have a role in HBO's adaptation alongside Troy Baker, who played Joel in the games.

In The Last of Us' trailer, Johnson's and Baker's roles were seemingly revealed, with the former hinting at Ellie's immunity through a longstanding game theory. That theory is that Ellie's mother was infected when she was pregnant with her, meaning Ellie was exposed to the Cordyceps infection as a fetus. This could mean that her immunity was instilled into her before her birth, and thus that when she was later exposed to the Cordyceps infection through the bite, it had no effect on her. This links to Johnson, as many have theorized she will be playing the role of Ellie's mother in The Last of Us.

In the trailer, Johnson can be spotted holding a newborn baby, with the trailer heavily implying that Johnson's character had just given birth to the infant. Johnson playing Ellie's mother in HBO's Last of Us video game adaptation would make sense, as it is both a neat link to Johnson's original game character and would establish Ellie's earlier life, which was not focused on in the game. This sequence could confirm that Anna, Ellie's mother, was infected while pregnant with her, which in turn caused her immunity.

While this is never officially confirmed in the game, it would make more sense than a simple mutation of Ellie's cells to repel the infection. One thing that is clear is that The Last of Us episode 1 established Ellie's immunity. Going into the remainder of The Last of Us, it will be interesting to see just how deep HBO's adaptation of the best-selling game will delve into this plot point, and whether it will officially confirm the means of Ellie's immunity to the infection that ravaged mankind.

New episodes of The Last of Us release every Sunday on HBO.

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