The Last of Us finale teases origin of Ellie's immunity: 'We don't answer that conclusively'

Warning: This article contains spoilers from The Last of Us season 1 finale.

The team behind HBO's The Last of Us, which includes one of the games' creators Neil Druckmann, sees the show more as a companion to the source material — and vice versa. As actor Troy Baker previously told EW, it's like they enrich each other. But the show also adds to some lingering questions gamers have had for years since that first title was released in 2013.

The Last of Us season 1 finale gave viewers an authentic adaptation of the first game's core ending, while also dramatizing Ellie's birth. How Ellie is immune to the fungal plague has been a question many fans have theorized about, but we finally get a major reveal, one that the creators say still doesn't solve the mystery "conclusively."

The opening sequence of episode 9 reveals Ellie's mom, Anna, who in a poetic move is portrayed in live-action by Ashley Johnson, the award-winning actress who originated the character of Ellie in the video games.

Liane Hentscher/HBO Ashley Johnson on 'The Last of Us'

Anna is pregnant with baby Ellie and is on the run from an Infected. She tears through the woods and seeks refuge in a remote farmhouse, which has served as her group's home base. Nobody is there, however, and Anna is forced to grapple with the Infected. She kills her attacker, but she's been bitten. At the same time, Anna has given birth to baby Ellie, whose umbilical cord is still attached to Anna. She frantically severs the cord, but the presumption is that the cordyceps infection was transferred to the infant.

Marlene (Merle Dandridge), Anna's life-long friend, arrives and is forced to kill Anna after promising to take Ellie with her group to Boston and keep her safe.

Later in the episode, we get one of Joel's (Pedro Pascal) final confrontations with Marlene before all hell breaks loose. She says the doctors working with the Fireflies believe the cordyceps has been growing inside Ellie since birth. It "produces a chemical messenger" that "makes normal cordyceps think that she's cordyceps," Marlene explains.

Speaking at a virtual press conference with reporters ahead of the finale, Druckmann says the Anna sequence "does hint and gives some theories as to why Ellie is immune, though we don't answer that conclusively."

A lost animated short

Liane Hentscher/HBO Marlene (Merle Dandridge) cradles a baby Ellie in the opening to 'The Last of Us' season 1 finale

Druckmann shared a "short" version of the origin story behind the Anna sequence with Johnson.

"When we were wrapping up the game, there were these opportunities to do other pieces of art or storytelling to help promote the game," Druckmann explains. One of those opportunities was American Dreams, a comic book set in the world of The Last of Us that told the story of Ellie and Riley. That arc was later adapted into Left Behind, a separate DLC (downloadable content) gamers could purchase and play through from Ellie's perspective.

Another opportunity was an animated short. "I wrote this short script about Ellie's mom and how she gave birth to Ellie, was bitten at the same time, wasn't sure if she was infected during that birth," Druckmann recalls. "It just became a little character drama that felt like it spoke to the same themes of parental love for their child and how much you're willing to do even when you're on death's door."

After the first deal for that short fell apart, Druckmann says there were conversations with a completely different company to develop the story as a new The Last of Us game. He confirms material was written to address the identity of Ellie's father, who has thus far remained absent from the games and the show. The story would have climaxed with the birth of Ellie, which is now the sequence that kicks off the season 1 finale episode of the HBO adaptation.

Liane Hentscher/HBO Bella Ramsey's Ellie in 'The Last of Us' season 1 finale

Speaking about Ellie's father, Druckmann says, "I'm reluctant to say anything about it because as I now found out several times, stories that I think are failures and will never see the light of day sometimes see the light of day."

When that game concept fell apart, Druckmann says he became interested in live-action and thought about making the Anna story into a short. He adds, "I was talking with Ashley Johnson about her starring in it, and we both got busy, so that fell apart."

Druckmann completely forgot about the idea until he started meeting with his fellow showrunner Craig Mazin to develop The Last of Us HBO series. More importantly than the possible explanation of Ellie's immunity, the sequence, Druckmann says, shows "the relationship between Marlene and Anna."

"When you get to the ending and we put Marlene against Joel and they have their own opposite philosophical terms of how to approach 'if the ends justice the means,' knowing how close she was with Anna and that Anna's dying wish was 'take care of my kid,' I think gives more weight and maybe more tragedy to the sacrifice Marlene is trying to make for the betterment of mankind," he adds.

What we know of Ellie's immunity origin from the game

Naughty Dog A note from Ellie's mom Anna can be found in 'The Last of Us' video game

There are a couple core details from the 2013 video game that also informed the Anna sequence, though they might've gone unnoticed to the casual player.

The first is a letter written to Ellie by Anna. Playing as Ellie in the "Winter" deer-hunting section, gamers can find the note in her backpack. It's scrawled on a scrap of notebook paper that is streaked with bloody fingerprints.

"Ellie, I'm going to share a secret with you," the letter begins. "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."

Anna writes that Marlene will look after Ellie. "There's no one in the world I trust more than her," she says. "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."

Naughty Dog Marlene's diary in 'The Last of Us' game offers more backstory to her relationship with Anna

At the end of the game, as Joel is weaving through the hospital killing Fireflies to find Ellie, players can find various tape recorders with messages Marlene as recorded for herself. You can also find a diary Marlene kept about her experiences. One such find is a voice memo she addresses to Anna.

"Despite everything I was in charge of, I looked after her," Marlene laments, knowing the doctors have to kill Ellie in order to make a vaccine. "I would've done anything for her, and at times... Here's a chance to save us, all of us. This is what we were after, what you were after. They asked me to kill the smuggler. I'm not going to kill the one man in this facility that might understand the weight of this choice. Maybe he can forgive me."

"Here, we get to dramatize it," Druckmann says of working this material into the show. "It's this beautiful thing because we get to do it with Merle and Ashley. So this relationship that has existed in Merle's mind when she was recording those lines [for the game], she got to see it live — and she got to see it with one of her good friends, with Ashley Johnson. There's something really nice about the actors that have contributed so much to the bringing-to-life of the story and them doing it again on the show."

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