In “The Last of Us,” the character Ellie is revealed to be immune to the Cordyceps fungus that has infected most of humanity and turned them into zombie-like creatures. It is later revealed that the character Ellie’s immunity is due to a unique mutation in her genes.
The game’s storyline explains that the Fireflies, a militant group attempting to restore order and find a cure, discovers Ellie’s immunity and believes that she holds the key to developing a cure for the Cordyceps infection. The player, controlling the character Joel, is tasked with escorting Ellie across the country to reach the Fireflies’ laboratory and hopefully develop a cure.
Throughout the game, the characters encounter numerous obstacles and challenges, including hostile survivors and infected enemies. Ellie’s immunity to the infection proves to be a crucial factor in their survival, as it allows her to interact with infected creatures without getting infected herself.
At the end of the game, it is revealed that the Fireflies plan to sacrifice Ellie in order to create a cure, which Joel opposes. He ultimately saves Ellie from the Fireflies and takes her away, effectively preventing the cure from being created.
In summary, Ellie’s immunity to the Cordyceps fungus is due to a unique mutation in her genes, which is the reason why the Fireflies believe that she is essential to finding a cure for the infection.