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

One of the greatest stories ever told in video games — and one of the best stories out there, period — was 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 adaptation arrived on HBO with massive shoes to fill.

As the 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 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?

A number of theories have been floated regarding Ellie’s immunity, but none were confirmed by Neil Druckmann or Naughty Dog up until The Last of Us concluded its first season on HBO. Thus, for years, it was 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, and 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.

This theory is certainly intriguing, but many felt it didn’t quite make sense in the broad context of the Last of Us story. See, if Ellie’s mother had been bitten while pregnant with Ellie, it’s very likely 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. Most feel there wouldn’t have been enough time, between Anna’s initial infection and Ellie’s birth, for any immunity to pass on to her.

Despite broad debunking of the theory that Anna passed immunity to Ellie, HBO’s adaptation of The Last of Us story seems to have officially confirmed the idea. It’s not explicitly clear that Anna’s infection gave Ellie her immunity, but the show certainly hints toward this idea, and many fans took it as official canon when Anna was bitten mere minutes before giving birth to the series protagonist. With this in mind, we can broadly accept that Neil Druckmann took the HBO adaptation as an opportunity to officially confirm the culprit behind Ellie’s unique condition, and accept that Ellie gained immunity through her mother’s bite, despite the fact that she was already full-term when the infection happened.

Druckmann has used the HBO series as an opportunity to clear up story holes and dig deeper into plot points the games didn’t allow him to explore, and the confirmation of Ellie’s immunity is just one of many. The show’s seeming confirmation of the culprit behind Ellie’s rare immunity opens up doors for fresh storytelling in the future, as it could allow for other immune people to emerge. There’s no way Anna was the only pregnant woman to ever get bit, and at least one or two babies were surely born post maternal infection. They easily could have died from various causes in the weeks, months, and years following their births, of course, but an official explanation allows room for more immune to arise in the future of the story.