'The Last of Us' finally explains exactly how Ellie became immune to the cordyceps infection

• Warning: There are massive spoilers ahead for "The Last of Us" season one finale.
• "TLOU" showed the origin of Ellie's immunity to the outbreak, something the game never showed.
• An infected bit Ellie's mom moments before Ellie was born, confirming a fan theory.

The finale of "The Last of Us" finally revealed exactly how Ellie Williams (Bella Ramsey) became immune to the cordyceps infection, confirming a long-standing fan theory.

In a flashback at the episode's start, we meet Ellie's mother, Anna (Ashley Johnson). As she's on the run from an infected person, she goes into labor and gets bitten in the moments leading up to Ellie's birth. 

The finale implies Ellie's immunity is linked to Anna's bite on her inner thigh, corroborating a fan theory that Ellie's built-in protection against the cordyceps outbreak was the result of her mother somehow getting bitten before childbirth. 

In the present, after doctors run tests on Ellie at the hospital, Marlene tells Joel the following:

"Our doctor thinks that the cordyceps in Ellie has grown with her since birth. It produces a kind of chemical messenger. It makes normal cordyceps think that she's cordyceps. It's why she's immune."

In other words, Ellie's body appears to have a mutation of the fungus that tricks cordyceps into believing she's one of them. Druckmann hinted to press there may be more to reveal there.

"It does hint at and give some theories as to why Ellie is immune, even though we don't answer that conclusively," Druckmann told press in a conference for the finale, according to The Hollywood Reporter.

Though the game vaguely references and explains Ellie's immunity to the infection, fans never knew the exact origin of her immunity until the show's season-one finale.

How Ellie's immunity is explained in the game

The 2013 video game of the same name never flashes back in time to show Ellie's birth or to introduce players to Anna.

Instead, players can read a letter that Anna left Ellie before she died.

It 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. 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!

The wording of the letter, which revealed Anna survived Ellie's birth, resulted in fan theories surrounding the exact cause of Ellie's immunity. For years, fans theorized Ellie's immunity was caused from a bite Anna received while pregnant or that Ellie's father was infected at the time of her conception. But the games never confirmed either to be the case.

In the game, players only vaguely learn a little more about Ellie's immunity once they arrive at the Firefly base in Salt Lake City, and tests are run on Ellie to understand why she didn't turn after receiving a bite in the mall with Riley.

Originally, Marlene tells Joel they believe the cordyceps growth inside of Ellie mutated.

"Once they remove it, they'll be able to reverse engineer a vaccine," Marlene tells Joel.

Afterward, Joel finds a surgeon's recording that says the cause of Ellie's immunity "is uncertain." 

Here's the transcription in full, which explains the Ellie's cordyceps far more thoroughly than the show: 

"Marlene was right. The girl's infection is like nothing I've ever seen. The cause of her immunity is uncertain. As we've seen in all past cases, the antigenic titers of the patient's Cordyceps remain high in both the serum and the cerebrospinal fluid. Blood cultures taken from the patient rapidly grow Cordyceps in fungal-media in the lab... however, white-blood-cell lines, including percentages and absolute-counts, are completely normal. There is no elevation of pro-inflammatory cytokines, and an MRI of the brain shows no evidence of fungal-growth in the limbic regions, which would normally accompany the prodrome of aggression in infected patients. We must find a way to replicate this state under laboratory conditions. We're about to hit a milestone in human history equal to the discovery of penicillin. After years of wandering in circles, we're about to come home, make a difference, and bring the human race back into control of its own destiny. All of our sacrifices and the hundreds of men and women who've bled for this cause, or worse, will not be in vain."

Now that "The Last of Us" season one has provided additional context to Ellie's immunity, it will be interesting to see if season two will continue to do the same.