Why did the Fireflies need to kill Ellie to make a cure in HBO’s ‘The Last of Us?’

Warning: the following article contains spoilers for The Last of Us season one finale, “Look for the Light.”

After nine heartbreaking and emotionally damaging episodes, HBO’s The Last of Us has finally concluded its first run of episodes with its season finale, “Look For The Light.”

In the shocking conclusion which line up with how the 2013 video game of the same name finishes up, we see Joel and Ellie finally arrive at the Firefly-occupied hospital in Salt Lake City. Then Joel discovers that in order for the Fireflies to manufacture a vaccine, they would need to kill Ellie. Rather than allowing his new surrogate daughter to have her life ended for the sake of the greater good (which would more than likely return the world of The Last of Us to a state of normalcy), Joel goes on a murderous rampage and takes Ellie back to Jackson, Wyoming. When her anesthesia wears off, Joel feeds Ellie a lie about scores of immune people who have had their blood tested for a cordyceps vaccine, but to no avail. He tells her that the Fireflies have stopped searching for a cure, which she begrudgingly accepts.

However, let’s turn the clock back slightly to Joel and Marlene’s pivotal conversation. Why exactly did the manufacture of a vaccine necessitate Ellie’s death? Allow us to break it down for you. 

Why Ellie had to die

While Marlene never states the Fireflies’ intention to outright kill Ellie on the operating table (in an effort to avoid Joel’s ire), she does explain to him the nature of Ellie’s cordyceps immunity. It has nothing to do with Ellie’s blood, as we may have observed with her completely futile (and completely incorrect) blood transfusion attempt on Sam’s wound. Due to the nature of her birth explained in the finale’s cold opening, the Firefly doctor believes that the cordyceps fungus inside her has been growing since birth.

As Joel points out after Marlene’s explanation, cordyceps grow inside the brain. Removing a sample of Ellie’s cordyceps in order to manufacture a cure would mean dissecting her brain, which, of course, would end Ellie’s life. The infection is completely intertwined with her brain, and the process of reaching and extracting a sample size large enough to manufacture the cure is impossible without more or less destroying it.