Why Isn’t All Humanity Sick in The Last of Us? Immunity Explained

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One of the things that we know about The Last of Us is that this live-action adaptation of a hit video game series may seem like a zombie show, but it actually isn’t. Instead, it deals with a pandemic that spread all over the world and was responsible for the devastation of the human race and the fall of civilization. In that regard, humans were infected, got sick, and attacked people. But why is it that not all of humanity is sick in The Last of Us?

Not all humans in The Last of Us are sick due to the fact that the Cordyceps Brain Infection can only spread through bites from infected people or when humans are exposed to the spores. As for Ellie, it is possible that she developed immunity when her mother was infected while she was pregnant with her.

The thing about The Last of Us is that there are still a lot of questions that are left unanswered, especially when it comes to video games. That’s because the story was always going to be about the survival of the characters involved instead of it having an overarching plot where humans are looking for a way to beat the infection. So, with that said, let’s look at why not all of humanity is sick in The Last of Us.

How Did The Outbreak Start?

It was never a secret that the storyline of HBO’s The Last of Us revolves around an outbreak that began in 2003 and stretched all the way to the events of the main storyline in 2023. That means that this pandemic lasted for 20 years and is set to last for an indefinite period of time, as the video game series suggests. Of course, it is the fact that this outbreak is so massive that the world fell apart in just a matter of days.


In the prologue, which takes place in 2003, we saw that the effects of the outbreak were so sudden that people began to act violently and attack other people. The neighbor of the Millers attacked Sarah without any warning whatsoever. And we saw that the entire Texas town where the Miller lived fell apart in a matter of hours, as an entire plane crashed into it and caused chaos. But how did the outbreak start?


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The video game series suggests that the outbreak began in South America. This outbreak is caused by the Cordyceps fungus, which is capable of attaching itself to insects and taking over its mind and body so that it can feed on the insect while controlling the insect’s body to feed as well so that it can continue to survive. 

It is important to take note that the Cordyceps fungus is actually real and that it does take over the brain and bodily functions of an insect. Here is a video of how the Cordyceps fungus works in nature:

As scary as the Cordyceps fungus may be, it is only capable of affecting insects in real life. It has no effect on humans whatsoever, and that’s why you don’t need to worry about it taking over your brain when you get exposed to it.

But in The Last of Us, the Cordyceps fungus somehow evolved and was able to take over human brains and bodies due to the evolution that it underwent. As mentioned, it evolved in South America during the events of the game. But in the HBO’s The Last of Us it evolved in Indonesia.

Regardless of where the Cordyceps Brain Infection (CBI) started, the spores of the fungus were quite possibly carried by-products that come from these tropical areas. For example, fruits and vegetables that were grown and brought to the United States may carry spores of these Cordyceps fungi.

As such, when humans were exposed to the spores, especially if they ingested enough of the products that carried them, they got their minds and bodies taken over by the Cordyceps fungus. Eventually, the infection spread so fast throughout major population areas throughout the world that human society fell apart and never recovered to its full glory in the 20-year gap between the start of the outbreak in 2003 and the main storyline in 2023.

Why Isn’t All Of Humanity Sick In The Last Of Us?

During the events of The Last of Us, there were still quite a number of survivors that were able to stay alive during the outbreak. Most of these survivors lived in Quarantine Zones (QZs) that were controlled by FEDRA, which was one of the few surviving arms of the United States government. And FEDRA was strict in making sure that people weren’t allowed to leave the QZs unauthorized and were always inspected due to the possibility that they could be infected.

Nevertheless, as hellish as the world may have become, there are still survivors that lasted through the 20-year gap between 2003 and 2023. So, why isn’t it that all of humanity is sick in The Last of Us?

In The Last of Us, the CBI could only spread if a person gets bitten by an infected or if a person gets exposed to the spores of the Cordyceps fungus. That is why, in the game, most of the people that wander outside the QZs have to wear gasmasks that reduce the chances of them inhaling spores of the Cordyceps fungus.


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Joel suggested in the prologue of HBO’s The Last of Us that the infection spread very fast in the city due to the fact that they may have gotten exposed to the products that carried the spores faster. As such, exposure to the spores was what led society to fall apart during the prologue of The Last of Us.

Meanwhile, there is Ellie, who appeared to have been bitten by an infected human but is somehow immune to the effects of the CBI. We saw this during the end of episode 1 when Tess was worried about the fact that her scans showed that she was infected, all while Ellie tried to explain that she was bitten three weeks ago but showed no signs or symptoms of infection.

tess and ellie scan

So, with that said, it is clear that Ellie is immune to the effects of the CBI, and that is why she is an important character to the world at that point in time. But why is it that Ellie is immune to the sickness, all while the rest of the world easily fell to the effects of the CBI?

In the game, it was never really explained why Ellie is immune to the CBI. But there are theories that point to the possibility that she may have developed this immunity before she was born. As such, the theories point to the possibility that her mother may have gotten infected during the time that she was pregnant with Ellie. So, even if her mother may not have survived the infection, Ellie developed immunity to it.

Regardless of why or how Ellie is immune, the thing is that the series creators are looking to introduce this story somewhere in the live-action series. Ellie’s mother and the reason why she is immune were never fully explored in the games, and that’s why the creators of the HBO version are looking to find a way to add it to the storyline so that more light could be shed on Ellie’s immunity to the CBI.

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