The infected are the result of a sudden outbreak of a mutant Cordyceps fungus that now affects human beings as opposed to insects. The fungus reportedly originated in South American crops; various newspapers in Austin in 2013 support this claim. The fungus infects the human brain, growing mycelium inside the brain tissue and killing the brain's cells. This erases their memories and drives them insane, modifying their instincts to seek no goal other than spreading the spores to others. Eventually the fungus kills its host, and will then grow out of the host's body to spread spores.