The Last of Us finale reveals how Ellie became immune from Cordyceps infection

Flashback sequence in episode nine explains mystery of Bella Ramsey’s character’s special power

Ellie’s immunity from the Cordyceps infection is finally explained in the closing episode of The Last of Us season one.

Throughout the first season of the HBO video game adaptation, fans have been trying to figure out how Bella Ramsey’s Ellie has been able to carry on living as a functioning human being after being bitten by the Infected.

Ellie’s immunity is like a special power, effectively turning her into precious cargo. It is the reason why she met Pedro Pascal’s Joel in the first place (the gruff antihero was tasked with safely smuggling her across the country so she could be used to find a cure).

While there is no explanation given in the original video game for Ellie’s immunity, the final episode of The Last of Us season one sees screenwriter Craig Mazin concoct a backstory for the character that explains it all.

***Warning: Major spoilers ahead for The Last of Us episode nine***

The episode opens with a flashback to a pregnant woman, Anna (Ashley Johnson, who played Ellie in the game via motion capture), running from one of the Infected.

She manages to reach a dilapidated old house in the middle of a field, but her Infected pursuer catches up with her, and attacks her as she goes into labour.

Anna knifes her attacker numerous times in the head, and seconds later a baby’s cry can be heard – she has given birth. She then cuts the umbilical cord with the very same knife and says to the baby: “Hello Ellie.”

However, we see on her thigh that she was badly bitten in the struggle.

After the opening credits, we see a young Marlene (Merle Dandridge) heading into the house.

Ashley Johnson as Anna in ‘The Last of Us’ (© 2023 Home Box Office, Inc. Al)

She finds Anna upstairs, holding Ellie and preparing to kill herself with the knife.

“I cut it before I was bit. Before,” she tells Marlene, lying about the chronology of the incident to try to save her baby from being killed. “I want you to take her with you to Boston and find someone to bring her up and make sure that she’s safe. And I want you to give her this.”

She gestures at the knife, which is the very same blade that we have seen Ellie carrying around with her all season.

Anna then begs Marlene, a lifelong friend, to kill her so that she cannot infect anyone else. Marlene resists at first, but then realises she has no choice and shoots Anna point-blank.

After the flashback sequence ends, the action rejoins Ellie and Joel on their journey across the country.

While it is left unspoken, the fact that Anna was bitten partway through giving birth to Ellie seems to be what has made her immune.

Read more about the finale in our review, here.