What is Cordyceps​ in The Last of Us?

A parasitic fungus illness known as Cordyceps brain infection (abbreviated CBI) causes an epidemic that transforms human beings into the infected monstrous creatures in The Last of Us series and video game franchise. The infection wreaks havoc around the U.S., causing people to quarantine themselves and kill those who have contracted the fungal illness. The Cordyceps brain infection in the series is based on the real-life Ophiocordyceps unilateralis.

Cordyceps in The Last of Us

A mutant strain of the Cordyceps fungus began spreading in the US in late September 2013. From South America, the cordyceps virus spread to people through contaminated crops. The Texas Herald, Joel and Sarah Miller's local newspaper in Austin covered the outbreak. The front page of the September 26 issue said: "Admittance spikes at area hospitals! 300% increase due to mysterious infection".

A few months later, the fungus, later known in medicine as Cordyceps brain infection, killed or affected almost 60% of humanity. The host goes through four stages of infection as the fungus grows while the host is still alive. About two days after infection, the host enters stage one, during which they lose their higher brain function and, along with it, their humanity, becoming hyper-aggressive and unable to think logically. The host reaches stage two of the infection within two weeks, during which the fungus starts to change their vision due to the damage to their visual cortex and the progression of fungal growth over their head.

After a year, the illness progresses to stage three, which causes blindness and facial scarring. In order to compensate, they start using a crude kind of echolocation. Rarely, the host reaches stage four if they live for more than ten years. They cover the majority of their bodies with hardened fungal plates. The host's body develops fungal projections that resemble stalks and release contagious spores when the fungus finally kills the host. Additionally, the infection can spread through live host bites. Only living hosts can become infected because the fungus is parasitic and cannot infect dead corpses, yet infected deceased can still release spores at any stage.

Survivors wear gas masks to protect themselves from airborne spores. Even if there are a lot of infected individuals in some outdoor locations, including the countryside, the infection doesn't seem to be able to spread there. The virus mainly flourishes in enclosed or underground spaces that humans tend to avoid, like sewers, subway tunnels, and some structures. Animals, particularly monkeys, may potentially contract the fungus. Even though the monkeys did not become infected, they may still bite someone and spread the infection.

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Cordyceps real-life inspiration in The Last of Us

The "Jungles" segment of the BBC documentary Planet Earth, which Sir David Attenborough narrated, served as the inspiration for the Cordyceps brain infection theory developed by Naughty Dog. The episode shows many insects and arachnids that were killed by multiple kinds of Ophiocordyceps unilateralis.

According to Neil Druckmann, the creative director, "it was all based on the idea that the more numerous a species becomes, the more likely it is to be preyed on by this fungus." Although people cannot become fatally infected with the Ophiocordyceps genera, they may experience behavioral changes or "disorders." Normal Ophiocordyceps would take weeks or months to become fatal because humans are easier to dissect (in terms of eliminating foreign chemicals or debris) than insects (also, body size can have a varied effect on the infection).