HBO's The Last of Us centers on a post-apocalyptic world overrun by infected humans. 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, and the main narrative kicks in, revolving around Joel and Ellie.
The Last of Us shows the post-apocalyptic life of the Boston quarantine zone through the eyes of Joel and Tess, who are introduced to Ellie, 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, early on in The Last of Us season 1, they establish why Ellie is so important to the Fireflies, and why they want her kept alive at all costs.
Ellie's Immunity Explained
She Is The Only One Who Isn't Infected The Same Way
The 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 the show. By the end of the season, it is revealed that Ellie's mom was bitten while she was still pregnant with Ellie, and then gave birth to her, which helped her be immune in some way.
Why Ellie Is Important To The Fireflies
They Need Her For A Cure
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, was so careful with Ellie by making sure she did not turn for a few weeks after being bitten.
Joel and Ellie attempt to travel to Salt Lake City, Utah, throughout the course of The Last of Us season 1. There is a major firefly base that hopes to use Ellie's immunity to find a cure. Despite the hope this could lead to a possible cure, Joel learns that surgery will kill Ellie, and he has to decide to save her or the entirety of humanity. In one of the show's most shocking moments, he fights his way through the Fireflies, eventually killing the doctor who is about to operate on Ellie and taking her to safety.
How Did Ellie Become "Immune"?
Her Mother Was Bitten While She Was Pregnant With Ellie
Throughout the entire first season of The Last of Us, Ellie's immunity is a mystery. In The Last of Us season 1 ending, the final episode explores a little more of Ellie's backstory as it shows her mom pregnant with Ellie. During the flashback, she is attacked by an infected while simultaneously giving birth to Ellie. While it is never actually explained exactly how Ellie became immune because of this, her being exposed to at least some of the infection could have been a way to build up an immunity in her system.
It's Ok That We Don't Learn Why Or How Ellie Is Immune
It's Still The Driving Force Behind The Plot And The Characters
While there is never a 100% concrete answer for why Ellie is actually immune, the details of that aren't actually what is important, especially with Joel saving her instead of letting her die in the hopes of finding a cure. Her immunity is more of a driving force for the entire plot, a way to get her and Joel from Boston to Salt Lake City, but the actual importance of the story comes from their character arcs and relationship rather than knowing that she is immune.
Ellie is willing to risk her life if it means a chance at saving humanity, while Joel can't fathom losing another child, something his journey with Ellie throughout the first season helped him work through. This leads to Joel lying to Ellie about her not being unique and that the Fireflies found dozens of people like her, and that they actually couldn't find a cure. While Joel might think the easy solution is to lie to Ellie, there is lingering doubt from her about what he is saying, which will be explored in The Last of Us season 2.
