'The Last of Us' Episode 9 reveals how Ellie is immune to the cordyceps infection

Ellie's immunity has always been a recurring theme but the series never really explains how or why she is invulnerable till the season finale
Bella Ramsey in 'The Last of Us' (IMDb)
Bella Ramsey in 'The Last of Us' (IMDb)

LOS ANGELES, CALIFORNIA : The truth about Ellie's immunity to the cordyceps brain infection that has devastated humanity is revealed in 'The Last of Us' Episode 9. The overarching storyline of 'The Last of Us' has centered around Joel delivering Ellie to the Fireflies because of her immunity to the infection that brought about the end of the civilized world with only a few pockets of humanity left, striving to stay alive since the first episode of the series. 

Ellie's immunity has always been a recurring theme in 'The Last of Us', but the series has never really explained how or why she is invulnerable. Questions have always been asked about why Ellie does not become infected like the rest of mankind ever since the bite marks she acquired in Episode 7's devastating conclusion. Read more to understand how Ellie's immunity actually works.

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Ellie's mother was bitten during childbirth

In the first scene of 'The Last of Us' Episode 9, Ellie's mother Anna (Ashley Johnson) is shown escaping the infected while still in labor. Anna hides in an upper room after sprinting to an empty house in the middle of nowhere that she had anticipated would be swarming with Fireflies. But an infected follows her and bursts through the door, forcing Anna to defend herself.

Anna kills the infected during the ensuing struggle, unaware that the stress of the incident had also forced her to give birth to a child, whom she names Ellie. Anna quickly cuts the umbilical cord that joined her to Ellie after realizing that she was bitten during the fight. Ellie's immunity from 'The Last of Us' is a result of being exposed briefly through her mother to the infection — a small enough dose for her body to develop an immunity response without succumbing to the disease. Since Ellie receives oxygen and nutrition through the umbilical cord, the cordyceps in Anna's system are also transferred to her. 

Marlene explains Ellie's immunity

Marlene reveals the real cause of Ellie's immunity to Joel in 'The Last of Us' Episode 9 around the halfway point. This information was uncovered by the surgeon of the Fireflies. The surgeon claims that the cordyceps fungi from 'The Last of Us', which has been a part of Ellie's body since birth, performs some sort of "chemical messenger" function. This messenger functions somewhat like a wall. When the cordyceps fungi cells enter Ellie's body, as it did when she was bitten in episodes 2 and 7, the existing cordyceps cells in her body 'trick' the new fungi cells into thinking she is already infected and renders her immune to its harmful effects.

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