‘The Last of Us’: How and Why Is Ellie Immune to Cordyceps?


The season 1 finale of The Last of Us answered questions lingering in the minds of fans of the video game and the live-action adaptation. Of course, we all know that Ellie is immune to the Cordyceps Brain Infection (CBI), and the entire story of the first season explores her journey with Joel to get to the Fireflies so they can use her to create a vaccine. But the final episode of the first season answered why she was immune. So, how and why is Ellie immune to the Cordyceps?

Ellie’s mother, Anna, was infected just after she gave birth to her and when Ellie was still connected to her through the umbilical cord. Ellie became immune because she grew with Cordyceps in her body. And the Cordyceps in her body was telling the infection that she was Cordyceps even though she wasn’t.

The fact that Ellie’s mother was infected when she gave birth to Ellie allowed Ellie’s body to develop immunity to the one thing that brought human civilization down to its knees. That is why she was very important to the Fireflies. However, as we saw in episode 9, Ellie would have had to pay the price to save humanity. Now, let’s look at how and why Ellie is immune.

Why Is Ellie Immune to Cordyceps?

The entire narrative of season 1 of The Last of Us focused on Ellie’s immunity to the Cordyceps Brain Infection (CBI), which brought civilization down. In episode 1, we learned that 20 years had passed since the outbreak started, but humanity was yet to find a cure or vaccine for the CBI, which means it was almost impossible for humanity to regain the old world.

But Ellie gave the Fireflies hope because they were the ones that were actively looking for a cure during a time when FEDRA was more concerned with the survival of the human race. That was when Marlene gave Joel and Tess the task of taking Ellie to their fellow Firefly members so that they could bring her to their base and use her immunity to develop a cure or vaccine that could quite possibly get rid of this outbreak once and for all.


Why Isn’t All Humanity Sick in The Last of Us? Immunity Explained

While they lost Tess along the way, Joel and Ellie continued their journey and managed to grow close to one another. However, the one thing neither Joel nor Ellie truly understood was why Ellie was immune to the effects of the CBI as she wasn’t turning into a Runner even though she was bitten and infected more than a few times already.

Episode 9, the season 1 finale, finally answered this question. It showed a flashback scene at the beginning of the episode as we met Ellie’s mother, Anna, who was seen running away from Runners while pregnant.

ellies mom

Anna defeated the Runners after her but gave birth due to stress. On top of that, she was also bitten during her fight against the Runners that were after her. And the problem was that Ellie was still connected to Anna, who was already infected with the CBI.

When Marlene and the other Fireflies got to the location, they saw that Anna was about to kill herself while Ellie was in her arms. She lied about how she cut Ellie’s umbilical cord before she was infected, allowing Marlene to believe that the child was not infected. And after Marlene promised to take Ellie to Boston, she killed Anna, a close friend of hers.

In that regard, Ellie’s immunity was a product of the fact that she was connected to her mother after her mother was infected by the CBI while fighting the Runners. However, Marlene knew nothing about this because she thought Ellie’s umbilical cord had been cut before Anna got infected. But it all worked out in Ellie’s favor because she became immune to the CBI, and it was only when Marlene met her again 14 years later that the Firefly leader found out about her immunity.

How Does Ellie’s Immunity Work?

After the Fireflies ambushed Joel and Ellie by mistake, the duo was taken to their base in a hospital in Salt Lake City. When Joel woke up from the ambush, he saw that Marlene was there to greet him. That was when Marlene explained to him that Ellie was being prepped for surgery due to the very nature of her immunity.

marlene and joel

As mentioned, Ellie was still connected to Anna through her umbilical cord when the CBI infected the mother. This meant that the fungus made its way to Ellie but in a manner that wasn’t enough to turn her. However, the Firefly doctors had a theory related to Ellie’s immunity.

Marlene said Ellie grew up with Cordyceps in her system since birth, and that allowed the Cordyceps to stay dormant in her body. And the fungus wasn’t enough to turn her into a Runner because it recognized Ellie’s body as Cordyceps. Ellie grew up with the fungus inside her body.

As such, whenever Ellie gets bitten by an infected, the Cordyceps in Ellie’s brain signal to the infection that Ellie is Cordyceps. This allowed the fungus to stop its invasion of Ellie’s body and brain before it happened. 


‘The Last of Us’: Why Did Joel Lie to Ellie?

Because of that, Ellie’s immunity was because she had Cordyceps in her brain. This is why her blood didn’t work on Sam when she tried to heal his infection in episode 5.

The fact that Ellie’s immunity was a product of her brain sending signals to the Cordyceps is why the Fireflies wanted to extract her brain from her body, thereby killing her. However, Marlene and the doctors believed they could create a vaccine allowing human brains to send the same signal to the Cordyceps whenever the inoculated person gets bitten. As such, the hopes of humanity lay in Ellie’s brain but at the cost of her life.

And as we all saw, Joel didn’t want Ellie to die without her consent, as this was what led him to kill all of the Fireflies, including Marlene and the doctors that knew what to do with Ellie’s brain to develop a vaccine for the CBI.