How is Ellie immune in ‘The Last of Us?’

Ellie from The Last of Us Part II
Image via Naughty Dog

One of the greatest stories ever told in video games — and one of the best stories out there, period — is set to be adapted for the small screen in early 2023. 

The Last of Us, a video game series from Naughty Dog, has been racking up awards for nearly a decade now, and every one was well deserved. The franchise tells a deeply heartfelt story about loss, perseverance, and grief, and the upcoming adaptation has massive shoes to fill.

As the HBO series starts to garner widespread attention, newcomers are joining longtime fans of the Last of Us story in pondering an overarching question. The story in the games — and soon the show — follows the gruff and capable Joel as he leans on hard-won smuggling skills to transport the teenaged Ellie across a post-apocalyptic U.S. The pair face constant danger during their journey, in hopes of using Ellie’s immunity to develop a cure for the virus ravaging mankind.

Ellie’s immunity is one of the core elements of The Last of Us, and largely drives the story onward. How she became immune, however, remains a mystery, even to longtime fans. She seems to be, in the Last of Us world, the only immune person out there, but how did she become immune to the Cordyceps virus?

How is Ellie immune?

Ellie Williams
Image via The Last of Us/Naughty Dog

A number of theories have been floated regarding Ellie’s immunity, but none have been confirmed by Neil Druckmann or Naughty Dog. Thus, it’s up to fans to come up with the most likely explanation.

The most popular theory out there leans on Ellie’s mother Anna, and the cause of her death. In the first Last of Us game, players spend the winter wandering the wilds as Ellie. Curious gamers found themselves able to sift through Ellie’s belongings and uncover a note written for the teen by her late mother. In the note, Ellie’s mother reveals that she is dying, and that Ellie is less than a day old.

This hidden fact quickly sparked fan theories about the nature of Anna’s death and its links to Ellie’s immunity. Quite a few fans out there believe that Anna was bitten, mere hours before Ellie’s birth, and the infection that ultimately took her life granted her young daughter immunity. Anna had just enough time — in the 48 hours before she turned — to pen the note for Ellie and take a moment to enjoy her brand-new baby before she succumbed to the virus.

Anna William's note
Image via The Last of Us/Naughty Dog

This theory is certainly intriguing, but it doesn’t quite make sense in the broad context of the Last of Us story. See, if Ellie’s mother had been bit while pregnant with Ellie, it’s very likely that the Fireflies would have connected the dots. Characters like Marlene are far from stupid, and they would have been racking their brains for any explanation behind Ellie’s immunity. If the explanation were as simple as Anna passing immunity to Ellie during gestation, they probably would have figured it out.

On top of that, Ellie would have been full term by the time her mother was bitten. There wouldn’t have been enough time, between Anna’s initial infection and Ellie’s birth, for any immunity to pass on to her.

A far more likely explanation — with this in mind — is simply that Ellie was born immune. There are rare cases of people being born with a natural immunity to a range of viruses and diseases in the real world, and Ellie is likely one of these people. She is an exceedingly rare person born with a mutation that allows her to sidestep the virus, and she simply wasn’t aware of this fact until she was bitten.

There are probably a few other people on the planet that share Ellie’s immunity, but they may have died through a variety of non-zombie related dangers. The post-apocalyptic world is deadly in more ways than one, and they could have died due to any number of accidents, violent attacks, or even due to a lack of basic medical care.