"The Last of Us" Presents a Theory For Why Ellie Is Immune to the Fungus

The first season of "The Last of Us" has been fueled by one quest: for Joel (Pedro Pascal) and Ellie (Bella Ramsey) to make it to the Fireflies to find out if her bizarre and perhaps unique immunity to the cordyceps fungus and brain infection can be used to bring about a worldwide cure. Joel was skeptical of this quest, but it was the dying wish of Tess (Anna Torv), so he takes on the task. Along the way, multiple people have died, adding to the sense of responsibility Ellie feels to help make a cure; she even tried to use her blood to cure Sam (Keivonn Woodard) when he was bitten.

"The Last of Us" video game never said exactly why Ellie was immune, though it did leave a major hint. The show made that hint canon in the season finale, illustrating Ellie's origins — but the reason why she might be the cure might just be a red herring. Ahead, we're breaking it down.

Why Is Ellie Immune to the Fungus?

At the start of episode nine, we see Ellie's mother, Anna (played by Ashley Johnson, who portrayed Ellie in the video game). She is moments away from giving birth, and she's running from an unseen foe while dealing with contractions. She runs into what she thinks is a safe house, calling out, "It's me," but the house is empty. She climbs the house's stairs and realizes her water has broken. She rests on the floor in an empty room, closing the door behind her. Her only protection is a knife, and an Infected zombie claws at the door, chasing her down.

She's able to stab the zombie and kill it — but not before it bites her. Anna also gives birth during the struggle and hears Ellie's cry right before she notices the bite on her leg. She cuts the umbilical cord and cradles her daughter, full of love and admiration and proud of how "tough" the baby is already.

Marlene (Merle Dandridge), the head of the Fireflies, appears that night and finds Anna and the baby. Anna tells her she was bit after she cut the cord and that she didn't nurse her, so the baby (who she's named Ellie) won't be infected. Anna begs Marlene to take Ellie to Boston, adding that Marlene must shoot her lifelong friend before she can become an Infected. Marlene eventually agrees.

In the present day, after Joel and Ellie have been taken by the Fireflies, Marlene explains to Joel their theory as to why Ellie is immune. "Our doctor, he thinks that the cordyceps in Ellie has grown in her since birth," she explains; apparently she eventually realized her friend lied about the timing of her infection. "It produces a kind of chemical messenger. It makes normal cordyceps think that she's cordyceps, it's why she's immune. He's going to remove it from her, multiply the cells in a lab, produce those chemical messengers, and then we can give it to everyone." They think it could be a cure.

But cordyceps grows in the brain, so to remove it, they'd have to kill Ellie. The Fireflies didn't tell Ellie before they took her into surgery, and Joel won't let them kill her.

Why Is Ellie Immune in the Video Game?

The video game has hinted at a similar origin for Ellie's immunity, though it's never spelled out. Ellie in the game has a letter from her mother in her backpack. The letter reads:


I'm going to share a secret with you, I'm not a big fan of kids and I hate babies. And yet . . . I'm staring at you and I'm just awestruck.

You're not even a day old and holding you is the most incredible thing I've done in my life — a life that is about to get cut a little short. Marlene will look after you. There's no one in this world I trust more than her. When the time comes, she'll tell you all about me. Don't give her too much of a hard time. Try not to be as stubborn as me.

I'm not going to lie, this is a pretty messed up world. It won't be easy. The thing you always have to remember is that life is worth living! Find your purpose and fight for it. I see so much strength in you. I know you'll turn out to be the woman you're meant to be.

Forever . . . your loving mother, Anna

Make me proud, Ellie!

Presumably, Anna wrote that letter in between when she was infected and when she died, with Ellie's birth either happening right before or right after she was infected. A separate question raised by this letter is how Ellie ended up in a military school instead of with the Fireflies for life.

Does the Explanation For Ellie's Immunity Make Sense?

As many fans have pointed out in Reddit theories over the years, this explanation doesn't 100 percent track. First, if Ellie had the infection from birth, why wouldn't she have set off FEDRA's virus scanners through the years? She would not have been able to attend a FEDRA school if she was scanning positive the whole time. And after she's bitten, she clearly believes she will turn into a zombie, so there's no indication that she has any idea she might be immune.

Others have speculated that because Ellie experienced a very small amount of the fungus at birth, her immune system built up a defense to kill it, the way a vaccine sparks in immune systems. But in the video game, we see through the surgeon's tests that though Ellie lights up fungal scanners, she doesn't have boosted immune activity that comes with fighting off an infection. For her white blood cell count to be normal, she would be testing negative for the fungus, but she isn't.

Others have theorized that this is all just a coincidence and that the fungus that Ellie was infected with had a mutation that meant everyone who was infected with that strand didn't turn. But Riley (Storm Reid) and Ellie were bitten by the same zombie and both turned, so that might not be true either.

The uncertainty in all this is maybe the point, though. Marlene doesn't know if Ellie is the cure, but she's willing to kill her for the chance that she might be. In a way, it all just emphasizes Marlene's desperation for an end to the infection.

Could Ellie's Immunity Have Led to a Cure?

From a scientific perspective, it's not clear that Ellie's immunity could have led to a cure or vaccine. In the real world, we still don't have any vaccines for fungus infections. NBC News reported in February that there are some in the works, but they won't be on the market until at least five years from now. And that's in a more technologically advanced and whole world than the postapocalyptic nightmare of "The Last of Us."

Vaccinologist Christine Rollier also said in a 2016 video about the video game for Wellcome Trust that if the infection were real, it would take 20 years of research to find a cure. Rollier also said in another video for the project that there's "not a chance" that tissue from Ellie could be used to make a vaccine. "That's not how a vaccine or the immune system works," she said. She said it's not that Ellie's immune system fought the infection off, it's that she's "not susceptible" to the fungus, which is not the same as being protected. She compared it to how some people can't get HIV because they have a mutation in a protein that the virus can't attach to. She did add that if Ellie had antibodies that fought the fungus, you could possibly use those to create more antibodies. But that's not what the Fireflies are even looking for in the game and show.