Maybe Ellie isn't immune to the Cordyceps virus in Last of Us 2 after all

In The Last of Us, Ellie's immunity to the Cordyceps virus that destroyed most of human civilization is the entire reason Joel is tasked with escorting her across the country: so the Fireflies militia can use her biology to develop a cure. An oft-forgotten Easter egg from the first game's final chapter, however, hints that her immunity isn't so clear-cut. By all accounts, Ellie sets of on a quest for revenge in the sequel after her community suffers a great loss, but what if all her rage is impacted by the growing influence of the Cordyceps infection?

The Cordyceps Brain Infection is a parasitic fungal infection that, once a host is infected (typically through a bite or airborne spores), quickly spreads to the brain and erodes higher brain function, rendering them hyper-aggressive within a matter of days. If they live long enough, the infected mutate into increasingly more grotesque monsters as the fungus takes over their body and grows armored plates, beginning with the head.

Nothing of the sort happens to Ellie over the course of the first game, and she was bitten weeks before the main narrative starts. One audio log from the hospital chapter of The Last of Us sheds light on one way Ellie might still be impacted by the virus.

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This theory is based around the Surgeon's Recorder audio log, which can be found within the hospital. Here's a full transcription:

April 28th. 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.

While the audio log is pretty optimistic about Ellie's condition, several lines stand out: "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." Infected people quickly become more aggressive because fungal growth within their bodies erodes higher brain function first.

Ellie has never shown any signs of fungal growth, so we're led to assume it never reached her brain, but this passage — through the difficult-to-parse jargon — seems to say otherwise: "antigenic titers of the patient's Cordyceps remain high in both the serum and the cerebrospinal fluid." In other words, the Cordyceps infection is present in her blood and in the fluid around her brain and spinal cord. But there are no signs of fungal growth and as a result, her body isn't actively fighting the infection.

What if, over the years, the Cordyceps infection has still infected her brain, causing Ellie to react with increased aggression in The Last of Us Part 2? Her rage-induced quest for revenge could become a full-on descent into madness because of the infection.

Many of the story trailers and interviews for The Last of Us Part 2 stress how Ellie is driven by revenge to a fault after someone she loves is hurt at the start of the game. Of course, there is real emotion at play here, and the grisly nature of this violent, post-apocalyptic world is very far removed from reality. Is something more than emotions influencing Ellie's brain?

Could the Cordyceps finally be getting to Ellie, influencing her decision-making? We don't know how her condition may have progressed during the five years between the first game and its sequel, and the doctor's log from the first game does state that her blood will grow Cordyceps in the right conditions.

While it's doubtful that The Last of Us Part 2 would transform Ellie into a Runner (the first primary stage of infection), we have to entertain the possibility that the infection could have symptoms, even for someone that appears immune.

The first game was purposefully vague about how Ellie's condition works, especially when Joel cut any experimentation short by freeing Ellie instead of letting the doctors resume their experiments. There's a lot of story potential that might be explored here, whether that happens in The Last of Us Part 2, a DLC for the game, or even a third game in the series. What if Ellie is highly resistant to the Cordyceps infection and not totally immune?

We have no way of knowing whether some version of this theory comes to fruition in the actual plot of The Last of Us 2. Ellie's brutal quest for revenge could be as simple as that, but we can't help but wonder about the impact the virus would have on her body and brain over extended periods of time. It's definitely something to think about as you begin playing the game very soon.

The Last of Us Part II will be released exclusively for PS4 on June 19, 2020.