Why Is Ellie Immune in ‘The Last of Us’? The Most Convincing Theories For Why She Can’t Be Infected by Cordyceps

Sophie Hanson
The Last of Us
Photo: Liane Hentscher/HBO

Warning: This article contains spoilers for The Last of Us. As episode one of HBO’s new post-apocalyptic series came to a close, viewers were left wondering why is Ellie immune in The Last of Us to a plague that decimated most of humanity. And that goes for those who have and those who haven’t played the critically acclaimed video game by the same name, developed by Naughty Dog.

For the uninitiated, The Last of Us is set 20 years after a devastating pandemic that wiped out most of humankind. The disease, a parasitic fungus known as Cordyceps, quickly overtakes the host’s body and mind. Horribly mutated and aggressive, they become what’s known as “the infected”. Joel (Pedro Pascal), a hardened survivor, is hired to smuggle Ellie (Bella Ramsey), a 14-year-old girl, out of an oppressive quarantine zone. What starts as a small job soon becomes a brutal and heartbreaking journey as they both must traverse the US and depend on each other for survival.

When Ellie reveals at the end of episode one that she was bitten by an infected weeks ago and hasn’t “turned”, we learn that somehow, she’s managed to gain immunity. But how? Here are the most convincing fan theories.

Why is Ellie immune in The Last of Us?

Why is Ellie immune in The Last of Us? We’re not entirely sure, though the original game released in 2013 might provide some hints. Before we dive into that, it’s helpful to understand Ellie’s backstory—and, it’s important to keep in mind that this is all information gathered from the games, parts One and Two, so HBO might take it in a different direction.

Ellie was born in the spring of 2019, by which time the cordyceps infection has spread throughout the United States and led to the collapse of civilization. There is no cure nor a vaccine and what’s left of humanity is restricted to authoritarian quarantine zones (QZs). Those that become infected are dealt with quickly, as we saw tragically in the young boy who was euthanized in episode one of the HBO show.

The Last of Us

Liane Hentscher/HBO

Her mother, a nurse by the name of Anna, died the day Ellie was born. Thus, Ellie was sent to an orphanage inside a Boston QZ. While at the orphanage, Ellie befriends the 16-year-old Riley. Adventurous and brazen, they sneak out to explore an abandoned mall and other pre-apocalypse relics like arcades. But one night, they’re ambushed by what’s called a runner, a human in the first stage of infection.

Both girls are bitten and while Riley quickly succumbs to the infection, Ellie survives, though she still carries a mutated form of the infection inside of her which is why she still tests positive for it. This immunity could help scientists discover a cure, which is why Joel must smuggle her out of the QZ to Salt Lake City to a team of researchers operating out of a rebel militia base.

Ellie shows no sign of being infected, though she is left with a small scar on her arm where she was bitten. There’s not much of an explanation as to how or why Ellie is immune, but one fan theory points to Ellie’s mother Anna and the cause of her death.

The Last of Us

Liane Hentscher/HBO

In “The Note from Mom” artifact that you collect in the video game while wandering the wilds as Ellie, Anna reveals she’s dying. Many fans believe Anna was bitten merely hours before Ellie’s birth and the infection that ultimately takes Anna grants her daughter immunity. The note reads:

“Ellie, 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.

She continues: “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!”

This fan theory seems unlikely, however, given that Ellie would’ve been full-term by the time Anna was infected, which doesn’t give enough time for immunity to be passed down to a newborn. A far more likely explanation is quite simply that Ellie was born immune. Given the outbreak was less than two decades prior to her birth, this likely means Ellie is an exceedingly rare person with natural immunity and a mutation that allows her to sidestep infection.

The Last of Us

Liane Hentscher/HBO

There may be others out there like her, but given how treacherous the post-apocalyptic world is, they may have died in a myriad of other ways, including violent attacks or just lacking basic medical care. Whether HBO’s show will delve more into the how or why of Ellie’s immunity remains to be seen. You’ll have to watch to find out!

Why is Ellie important in The Last of Us?

Why is Ellie important in The Last of Us? Ellie is special in The Last of Us because she’s immune to the fungal plague that has wiped out most of humanity. We’re not sure if she’s the only one immune, but she certainly seems like one of the very, very few people that are.

What are the infected in The Last of Us?

What are the infected? They’re humans who have succumbed to the Cordyceps brain infection and horribly mutated into new forms as the fungus continues to take over. Calling them zombies wouldn’t really be doing them justice, though, nor is it really accurate as “the infected” aren’t reanimated corpses but are living hosts hijacked by this parasite. As with most infections, its single goal is to reproduce and in The Last of Us, this can be done in two ways: through a bite or through inhaling spores that move in the air.

The infection cannot be removed from the host without killing them given how quickly it spreads to the brain. Neil Druckmann (co-president at Naughty Dog) and his team of artists went to “great lengths to create a full biological cycle for these things” and the level of detail is astounding—spanning its advancement from the first few days of infection to the final stage, wherein the fungus has completely taken over the body and, when it feels like it’s time to die, it finds a space where it can become a part of the environment, spewing spores in the hopes that a new host will come along to infect.

The Last of Us is available to stream on HBO Max. New episodes are released Sundays at 9 p.m. ET. Here’s how to watch it for free.

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