Ellie is immune to the deadly Cordyceps brain infection in The Last of Us TV show most likely due to being born with a mutation that makes her unaffected by the parasitic condition.
The exact way Ellie became immune to the world-ending infection is never officially revealed in The Last of Us games.
However, there are some hints we can draw from:
The theory that is most likely correct is that Ellie was simply born immune to Cordyceps in the same way that some people are born with natural immunity to other viruses and diseases.
Thanks to incredibly rare mutations, we’ve seen people in the real world who were born immune to conditions such as HIV, meaning that this explanation is very plausible.
It’s even likely that Ellie isn’t the only person in the world of The Last of Us who is immune to the Cordyceps brain infection. However, seeing as immunity can only be tested after getting infected and surviving the encounter, many people likely didn’t discover their own immunity.
It’s also worth noting that HBO is changing one major aspect of the Cordyceps infection in The Last of Us show vs the games! Whether or not that impacts Ellie’s immunity is unclear.
Note: The following section contains spoilers for The Last of Us video game.
Was Ellie’s Mother Immune in The Last of Us?
Some fans have theorized that perhaps Ellie’s immunity came from her mother, Anna, who died shortly after the girl was born. However, there is a significant amount of evidence to dispute that claim.
It’s worth noting that in The Last of Us video game, a surgeon’s tape recorder indicates that Ellie’s white blood cell count does not spike, despite the presence of Cordyceps in her bloodstream.
Read More: Where to Watch The Last of Us TV Show
From that information, we can glean that the character’s immune system is not fighting off the virus in the same way that our bodies would battle the flu after a flu shot.
Instead, Ellie is naturally immune, which puts to rest theories that suggest that perhaps she was infected as a fetus and developed an immune response.
Similarly, if Ellie’s mother was bitten while pregnant with her child, she would have had to be in the very last stages of pregnancy in order to give birth before dying – far too late for any immunity to be developed.
And on top of that, Marlene and the rest of the Fireflies are aware of Ellie’s condition.
If her mother was bitten right before her birth, the group wouldn’t find it too hard to work out exactly how the immunity occurred, rather than being confused by it throughout the series.
For more information, check out the full The Last of Us episode release date, cast list, trailers and more here!