The following contains spoilers for The Last of Us Season 1, Episode 9, "Look for the Light," which aired Sunday, March 12 on HBO.

The Last of Us hasn't avoided explaining the science behind its mushroom-infested pandemic. The first three episodes detailed the origins of the Cordyceps fungal infection and how a mixture of flour and climate change possibly jump-started the end of the world. The HBO series explored the virus' nature again in its season finale by explaining why Ellie is immune to the infection.

The young girl discovered she was immune after being bitten by a lone infected in an abandoned mall, and after weeks of no symptoms, she became a rarity in the world. Even if The Last of Us occasionally lacks enough danger from infected to show why a vaccine is necessary, the show's entire story is founded on the journey to find a cure. That's why Season 1, Episode 9, "Look for the Light," goes back to the beginning to show how immunity is possible.

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The Science Behind Ellie's Immunity

"Look for the Light" opens on a flashback of Anna (played by original Ellie actor Ashley Johnson), Ellie's mother, who is on the run from infected. Hiding out in a barn, Anna deals with the pains of labor while fighting off an infected --.but she gets bit on the leg in the process before being able to cut the umbilical cord. To ensure that the infection doesn't spread to her daughter, Anna refuses to breastfeed Ellie and even considers committing suicide when it seemed like no one was coming for help.

Marlene and the other Fireflies eventually find Anna, who tells Marlene that she cut the cord before she was bitten... which wasn't true. But Anna took a chance that Ellie would survive, and she did. Now 14 years later, Ellie is the only known person to be immune to the Cordyceps infection because -- as Marlene explains via the doctor's examinations -- the infection she was exposed to at birth sent a "chemical message" to the Cordyceps that entered her body when she was bitten. The fungus Ellie was born with disguised her as Cordyceps, making it impossible for her to become infected.

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Are There Other Immune People in The Last of Us?

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The biggest question out of The Last of Us' immunity reveal is if other people can be born immune as well. Who's to say this hasn't happened to someone else since Ellie's birth? The problem is that nobody knows they're immune until get bit, which no one's willing to take a chance on. And it's not public knowledge that people can become naturally immune through these unusual circumstances because Anna lied about cutting the umbilical cord.

There's a very real chance that Ellie isn't the only immune person in the world. She just happens to be the only person who knows it. It's also likely that only people under 20 could be immune because they had to have been born after the outbreak. Yet there's enough about the science left vague for The Last of Us Season 2 to explore. Marlene and the doctor don't survive long enough to explore more of Cordyceps' dormant nature in a person, especially since they're in the dark about the odd circumstances of Ellie's birth. For now, though, it's a satisfying answer to a long-debated theory, and fans can rest easy that Ellie's immunity isn't some sheer coincidence.

The Last of Us Season 1 is streaming on HBO Max. A second season is in development.