The Secret Behind Ellie's Immunity Is Officially Revealed in the 'Last of Us' Show

Spoiler alert! This article contains plot details for the season finale of The Last of Us.

Joel and Ellie have reached the end of their journey in Season 1 of The Last of Us. The popular HBO show adapts the critically-acclaimed PlayStation video game of the same name and follows the survivors of a monstrous Cordyceps fungal infection that has decimated the United States.

Twenty years after the initial outbreak, Joel Miller (Pedro Pascal) is a quarantine zone smuggler who is deeply traumatized by the death of his daughter within the first hours of the virus spreading. He is charged with escorting a teenager named Ellie (Bella Ramsey) across the country in the hopes that her immunity to the virus could help save the world once and for all.

But why is Ellie immune to the Cordyceps in the first place? The show goes into more explicit detail than the game.

Typically, when someone is bitten by an Infected in The Last of Us, the Cordyceps takes over their body in a matter of days and effectively turns the victim into a monster. Miraculously, that isn't the case with Ellie. Despite being bitten on her right arm, the infection does not take hold of her body. It's for this reason that Fireflies rebel leader Marlene (Merle Dandridge) tasks Joel with taking Ellie to find a doctor who can research her immunity.

Interestingly enough, the show explicitly discusses where her immunity came from. The first moments of Season 1, Episode 9 — "Look for the Light — are a flashback depicting Ellie's mother, Anna (played by Ashley Johnson, who portrayed Ellie in the original games). Still pregnant with Ellie, Anna runs from an Infected on her way to a rendezvous point to meet with her fellow Firefly rebels, including Marlene.

She is the first to arrive and attempts to block the doorways in the house to keep the Infected away as her water breaks. Unfortunately, an Infected bursts its way through her barricades just as Anna has prepared to push. After a brief struggle, she manages to kill the Infected and gives birth to Ellie mere seconds after doing so. However, Anna discovers that she was bitten on her right thigh before she is able to cut the umbilical cord. Nevertheless, she cuts it and holds Ellie for as long as she can.

Marlene arrives sometime after the fact, and Anna lies and tells her that she cut the cord after she was bitten. She urges Marlene to kill her before she turns and to take care of Ellie in her absence.

In all likelihood, Ellie inadvertently received her Cordyceps immunity after her mother was infected. She received a dose of the virus through Anna's amniotic fluid where it developed within her body.

According to Marlene, her Firefly doctors theorize that because the Cordyceps were with her since birth, the virus in her system sends out a chemical message. This message supposedly communicates with foreign Cordyceps, making the virus believe that Ellie herself is already infected. With enough time and research, Ellie's immunity could even be reverse-engineered into a cure.

Folks who played the original game may have already gotten this hint. During the segment in which you play as Ellie in the winter, players have the option of searching through her backpack and finding a farewell letter from Ellie's mom. The letter itself implies that she was infected before giving birth to her, which would explain her immunity.

However, the show takes it a step further by revealing exactly what happened to cause Ellie's unique condition.

Season 1 of The Last of Us is now streaming fully on HBO Max.