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The Cordyceps Brain Infection (also known as CBI for short or the Infection) is a fictional disease in The Last of Us series. It is a parasitic fungus that has nearly wiped out mankind globally.
In The Last of Us, the mutant infection began to spread in the United States in late September 2013. In just several months, roughly 60% of humanity was either killed or infected by the cordyceps fungus, referred to as Cordyceps Brain Infection. The fungus grows while the host is still alive, taking away their higher brain function (and with it, their humanity), and causing the infected host to become hyper-aggressive and incapable of reason or rational thought (stage 1), stage 1 starts anywhere in between 1-2 days of infection.
As the infection progresses, the infection begins altering their sight as a result of progressing fungal growth over the head and corruption of their visual cortex (stage 2), stage 2 starts anywhere in between 1-2 weeks of infection. The infection will eventually scar their face completely, causing them to lose their sight, resulting in them developing a primitive form of echolocation to compensate (stage 3), stage 3 starts anywhere in between 1-2 years of infection. Over a very long time, they will eventually develop hardened fungal plates over most of their body (stage 4), stage 4 starts anywhere in between 1-2 decades of infection. If the fungus kills the host, the host's body will grow stalk-like fungal projections which release infectious spores. The infection can also be spread through bites from living hosts. Hosts can only be infected while alive, as the fungus is unable to infect dead bodies due to it's parasitic nature, though dead Infected can release spores regardless of stage.
To protect themselves from airborne spores, survivors wear gas masks; Joel has one strapped to his hip as does Tess. Obviously, those who are immune do not need them, namely Ellie.
The infection seems unable to spread in open air areas, such as the countryside, although some of these areas such as Lincoln and a suburban neighborhood in Pittsburgh have been shown with a high population of Infected. The infection mostly thrives in underground or enclosed areas commonly avoided by people, especially sewers, subway tunnels, and some buildings.
In the Left Behind DLC, during her search for medical supplies, Ellie discovers that an attempt was made by Captain Regan Francis to stop the infection from spreading throughout the body of her fellow survivor Private Eugene Ellis by quickly amputating his bitten right arm. Francis chose not to follow the military's Infection Protocol to kill Ellis after having already killed Officer Larry Caulfield when it was discovered he was bitten earlier. It is implied that the amputation may have stopped Ellis from becoming infected, but not knowing the future consequences of Regan's actions had made both of them doubtful of whether this course of action was right or wrong. Ellis later shot Regan dead out of fear that she would kill him, and later bled out in an air duct after escaping from some Infected.
- In reality, the Cordyceps fungus is only fatal to insects, not humans.
- It is unknown of how the mutant fungus was created. At the beginning of the game, it is reported that in a newspaper at Joel and Sarah's house, that the infection spread to humans due to infected crops from South America. But this still remains unknown of how the CBI was created.