‘The Last Of Us’ Mystery: Why Is Ellie Immune?

Ellie is the character at the center of 'The Last of Us.' You might be wondering: how is she immune to the infection that has obliterated mankind in the series? Here's what we know so far.

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Ellie Williams, played by Bella Ramsey, might just be the key to saving the world. In The Last of Us premiere, the world was ravaged in 2003 by the Cordyceps infection, a mind-controlling infection caused by a fungus that invades the host’s brain. Humans are unable to escape the wrath of the infection. Twenty years later, the world is a shell of its former self.

Civilization has been decimated by the Cordyceps infection and the subsequent global pandemic. Non-infected survivors are kept in authoritarian quarantine zones. There is no cure or vaccine available, even after all this time. When a child shows up at one of the quarantine zones, the boy is euthanized when officers learn that he’s infected.

Bella Ramsey
Tess looks at the scar on Ellie’s arm. (TLC)

Ellie is told by Marlene (Merle Dandridge), leader of the Fireflies, that she has a “greater purpose” than anyone could have ever imagined. Turns out, Ellie is immune to the Cordyceps infection. At the end of the premiere episode, Ellie reveals that she was bitten by one of the infected weeks ago and didn’t get sick. In episode 2, Ellie is bitten again but doesn’t get sick, unlike Tess.

Why Is Ellie Immune?

The answer to why Ellie is immune has yet to be fully explained in the show. Ellie was born in 2019 when the Cordyceps infection had already spread around the world. The Last of Us series begins in 2033 after brief flashbacks to 1968 and 2003.

Ellie could have been exposed to the Cordyceps infection in vitro, and that’s what led to her immunity. Ellie’s mom Anna, played by Ashley Johnson in the HBO series, died shortly after Ellie was born. Anna is never seen in the game, but she will be featured in the show. Naughty Dog’s Neil Druckmann, who created The Last of Us, told Newsweek, “There was a story that I had written after the game came out—about Ellie’s mom, that was going to be realized in this one way, and then it just kind of fell apart so it was always just sitting on a shelf.” Getting some of Anna’s backstory could provide insight into how Ellie is immune.

Bella Ramsey
Bella Ramsey as Ellie. (HBO)

After her mother’s death, Ellie is sent to an orphanage inside a Boston quarantine zone. Ellie becomes friends with Riley, who will be played by Storm Reid. One night when they sneak out, Ellie and Riley are bitten by one of the infected. While Riley dies from the infection, Ellie survives. However, she still has a scar from where she was bitten (as seen in the premiere) and continues to test positive for the infection despite not being sick.

Newsweek also pointed out a clue regarding Ellie’s immunity in the game’s epilogue. Players can find a “surgeon’s recorder,” and it reveals that Ellie’s condition is “an anomaly that scientists have not seen before in any of their previous subjects.”

Ellie’s immunity could be very simple — she could have just been born immune. Regardless of how she became immune, she is just about the most important person in The Last of Us world. That’s why Joel (Pedro Pascal) winds up trying to get her to Salt Lake City to researchers who want to try to find a cure or a vaccine.

How Did The Infection Spread?

The Last of Us series explained the origin of the Cordyceps infection in episode 2. The episode begins with a flashback to September 24, 2003. Ibu Ratna, a professor of mycology at the University of Indonesia, is asked to examine a specimen of ophiocordyceps that came from a human.

She looks a the leg of a corpse that has a bite mark on it. The bite came from a human. Professor Ratna then opens the mouth of the corpse and sees the live fungus in her mouth. The officer with Professor Ratna reveals that this all started 30 hours ago in a flour and grain factory in Jakarta, which Ratna calls a “perfect substrate.”

The Last of Us
A glimpse of the infected. (HBO)

The woman who was bitten got violent and bit three co-workers. The police arrived and killed her. The people she bit were later killed after they got violet as well.

The officer says Professor Ratna is here to stop this from spreading with a vaccine or medicine. “I have spent my life studying these things. So please listen carefully. There is no medicine. There is no vaccine,” she warns. Ratna says the only option is to bomb the city and everyone in it.

In the premiere during the 2003 portion of the episode, Sarah is originally going to make pancakes for breakfast, but she doesn’t have any flour. As Joel and Sarah head out for the day, their neighbor offers them biscuits as he feeds his elderly mother. Joel and Sarah politely decline the offer. After school, Sarah goes to the neighbor’s house to bake cookies. Sarah doesn’t eat the cookies because they’re not chocolate chip cookies. When Joel gets home from work, he realizes he forgot to grab the birthday cake.

In a theory post, Gamespot pointed out that the “common ingredient” in all of these food items is flour. During breakfast, the news talks about trouble in Jakarta. Jakarta, the capital of Indonesia, is home to the world’s largest flour mill. Contaminated flour is “likely the cause of the Cordyceps infection.”

In a post-episode 2 interview, showrunners Craig Mazin and Neil Druckmann confirm that contaminated flour is the source of the infection. “I think it’s pretty explicit,” Craig told Variety. Neil added, “Yeah, we pretty much said yes.”

The Cordyceps infection has four stages: runner, stalker, clicker, and bloater. As the infection evolves in a host’s body, they become more dangerous. When the infected die, the fungus continues to grow and spread wherever the corpse lies.

The premiere episode starts with a flashback to 1968 when an epidemiologist warned about the dangers of fungi. He said that there are some “fungi who seek not to kill but to control” and “fungi can alter our very minds.”

At the time, fungi couldn’t survive in humans. However, the epidemiologist noted that if the world got “slightly warmer,” then the fungi could adapt. He mentions certain fungi, including Cordyceps, could “become capable of burrowing into our brains and taking control not of millions of us, but billions of us. Billions of puppets with poisoned minds permanently fixed on one unifying goal: to spread the infection to every last human alive by any means necessary.”

He added, “And there are no treatments for this. No preventatives, no cures. They don’t exist. It’s not even possible to make them.” His words were almost like a premonition. Just 35 years later, fungi would be the cause of the world’s destruction.

Until The Last of Us series reveals the reason behind Ellie’s immunity, it will remain a mystery. Keep the theories coming! The Last of Us airs Sundays on HBO.