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We Now Know How Ellie Became Immune to Cordyceps in HBO’s ‘The Last of Us’

A pregnant woman in a yellow dress and leather jacket leans against a tree in the woods, looking frightened.

The season 1 finale of The Last of Us on HBO Max opens with a harrowing scene: a pregnant woman in labor runs through the forest and hides in an abandoned house, propping a chair against the door to keep her pursuer out. We soon see that that pursuer is one of the Infected, and as the woman groans through a strong contraction, it bursts in to attack her. They struggle, she stabs it in the neck to kill it, and then looks down to see that she’s given birth.

She also sees that she’s been bitten.

We find out that this woman is Ellie’s mother, Anna (Ashley Johnson), and the newborn is Ellie herself. Thanks to this scene, we now know how Ellie became immune to Cordyceps.

How Ellie is first exposed to Cordyceps

After she sees that she’s been bitten, Anna turns her attention to the new baby. She takes the knife she used to stab the Infected—the same knife Ellie carries with her throughout the rest of the series—and uses it to cut the umbilical cord.

Does she cut the cord quickly enough to prevent Cordyceps cells from her bite from entering Ellie’s bloodstream? It’s hard to say. Blood circulates pretty fast, but the placenta, which acts as a barrier between the parent’s body and the baby’s, might block the Cordyceps—or at least slow it down.

However, by using the Cordyceps-infected knife to cut the umbilical cord, Ellie’s mom ensures that Ellie will be exposed to at least a trace amount. For the first few minutes after giving birth, the umbilical cord continues to pump blood from the parent’s body to the baby’s. After that, it goes slack, and the placenta detaches from the uterus. For those first few moments, though, the umbilical cord is actively delivering a blood supply to the baby’s body, giving the Cordyceps cells on the knife an excellent chance of infecting Ellie.

When Marlene finds Anna, Anna lies about the sequence of events, telling Marlene that she cut the cord before she was bitten. It’s a desperate effort to save Ellie’s life, even though Ellie has been exposed to the infection—and it ends up being the reason Ellie becomes immune.

Related: What Is Bella Ramsey’s Height in HBO’s The Last of Us? on Twinfinite

Ellie’s immunity to Cordyceps, explained

14 years later, when Joel and Ellie finally reach the Firefly lab where doctors can use Ellie’s immunity to create a cure, Marlene explains what their doctors have found. According to Marlene, Ellie has had Cordyceps growing inside of her since birth. The Cordyceps produce a chemical messenger that tricks new Cordyceps cells into thinking that Ellie is also Cordyceps, meaning that they don’t try to take over her body.

Obviously, this explanation raises as many questions as it answers. Why didn’t the Cordyceps just infect Ellie completely as a newborn? Why does this mean they have to operate on her brain instead of just taking, say, a blood sample?

But, at least we now have a basic explanation for Ellie’s immunity. In the tumult and chaos of giving birth while fighting off an Infected, Anna accidentally gave Ellie a bigger gift than she could have possibly known.

(featured image: HBO Max)

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Julia Glassman (she/her) holds an MFA from the Iowa Writers' Workshop and covers film, television, and books for The Mary Sue. When she's not making yarn on her spinning wheel, she consumes massive amounts of Marvel media, folk horror, science fiction, fantasy, and nature writing. You can check out more of her writing at, or find her on Twitter at @juliaglassman.