The Cordyceps brain infection (abbreviated as CBI) is a parasitic fungal infection that has devastated mankind in The Last of Us series.

In The Last of Us, a mutated strain of the Cordyceps fungus began to spread in the United States in late September 2013. The Cordyceps originated from South America, carrying into humans through infected crops. Joel and Sarah Miller's local newspaper in Austin, the Texas Herald, reported on the infection, stating on the front page of the September 26 edition: "Admittance spikes at area hospitals! 300% increase due to mysterious infection" followed by the article "FDA expands the list of contaminated crops. Massive recalls anticipated" saying:
“The Food and Drug Administration's investigation of crops potentially tainted with mold continues across the country. Initial lists distributed to vendors nationwide warned against crops imported from South America, but now the scope has extended to include Central America and Mexico. Several companies have already voluntarily recalled their food products from the shelves.
―Texas Herald, September 26, 2013.”
Several months later, roughly 60% of humanity was either killed or infected by the fungus, later medically referred to as Cordyceps brain infection.
The fungus grows while the host is still alive, with hosts undergoing four stages of infection. Stage one begins within two days of infection, wherein the host loses their higher brain function (and with it, their humanity), rendering them hyper-aggressive and incapable of reason or rational thought. Within two weeks, the host enters stage two of the infection, wherein the fungus begins altering their sight as a result of progressing fungal growth over the head and corruption of their visual cortex. After a year of infection, the infection enters stage three; scarring their face and blinding them, resulting in them developing a primitive form of echolocation to compensate. In very rare cases, if the host survives for over a decade, they reach stage four. They develop hardened fungal plates over most of their body. When the fungus kills the host, the host's body grows 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 its parasitic nature, though dead infected can release spores regardless of stage. To protect themselves from airborne spores, survivors wear gas masks.
https://thelastofus.fandom.com/wiki/Cor ... _infection