'The Last of Us' finale finally explains why Ellie is immune

As well as beating us over the head with yet another intense volley of emotional punches, The Last of Us finale also answered a key mystery that's been hanging over the show since episode 1: Why, exactly, is Ellie (Bella Ramsey) immune? What makes her unique?

Ellie's past, in general, has been a bit of a mystery since she first linked up with Joel (Pedro Pascal). Episode 7, which took us back to Ellie's time in FEDRA school and a night with her friend Riley (Storm Reid) at an abandoned mall, explained how she got the now-healed bite on her arm — but episode 9 goes even further back, and in doing so it shows us the origin of Ellie's unusual gift.

How does Ellie first become immune to cordyceps?

In the opening scene of the finale, we see a heavily pregnant woman (Ashley Johnson, who originally played Ellie in the games) fleeing Infected and hiding in the upstairs of a large country house after her water breaks. As she slumps against the wall an Infected breaks down the door and charges at her. She manages to fight it off, but after killing it she looks down to see that her baby has arrived — and she's sustained a bite during the attack.

When she holds up her newborn and says the name "Ellie", we suddenly know exactly who it is we're looking at: Ellie and her mother, Anna. We also know Anna is lying when Fireflies leader Marlene (Merle Dandridge) arrives soon afterward and Anna tells her she cut Ellie's umbilical cord before she was bitten. She is not lying, however, about the fact that she chooses not to nurse Ellie for fear of infection.

Clearly, some of Anna's fresh infection passed to her daughter before the cord was severed — but not enough to actually infect Ellie herself.

So how does Ellie's immunity actually work?

It's not until halfway through the finale that we learn how this unusual birth affected Ellie physically. After finally making it to the Fireflies' hospital base in Salt Lake City that's been their destination all along, Joel wakes to find Marlene standing over him. Ellie, he's told, is already being prepped for surgery — and the Firefly doctor believes he knows why she's immune.

"Our doctor, he thinks that the cordyceps in Ellie has grown with her since birth," Marlene explains. "It produces a kind of chemical messenger. It makes normal cordyceps think that she's cordyceps; it's why she's immune. He's going to remove it from her, multiply the cells in a lab, produce those chemical messengers...and then we can give it to everyone. He thinks it could be a cure, Joel."

So in a similar way to how a vaccine introduces a small quantity of a disease into a person so they can develop the antibodies to fight it, it sounds as though Ellie already has her own, dormant version of cordyceps — and it's stopping her from catching the form that turns people into killing machines. The problem? Cordyceps takes over the brain, so Ellie's involvement in developing a cure requires her sacrifice.

Do we know this is exactly how Ellie's immunity works, for a fact? No, because the surgeon who would have removed Ellie's brain is permanently stopped in his tracks by Joel. But it certainly sounds viable enough. The problem is, if the Firefly doctor is right, Ellie's brain could literally hold the cure for cordyceps. But while she's still alive, humankind won't be able to access it.

The Last of Us is now streaming on HBO Max. New episodes air every Sunday at 9 p.m. ET on HBO.