Naughty Dog's PR machine made a lot of noise over the past year about how its apocalyptic infection is based on a real species of fungus, Cordyceps, that mind-controls ants, turning them into zombie slaves. It's both disingenuous and too modest. Though the infection in The Last of Us does in some ways mirror the Cordyceps species, in reality the infection's symptoms and ability to control hosts is an amalgam of several different parasites. However, what's truly impressive is that Naughty Dog's fictional parasite has an identifiable life cycle that both holds up to internal logic and retains just enough plausibility to make it scary.
In The Last of Us, a novel Cordyceps fungus makes the jump from insects to humans, quickly wiping out 60% of the world population. For both crafting visuals and creep-out factor, using the fungus was a master stroke. Cordyceps is a genus that includes at least 400 known species of fungi. Each of those Cordyceps species has evolved to target a single species of insect as its host - some thrive only in bullet ants, some in caterpillars, and others target other fungi - but all of them do roughly the same thing. When the fungus spores land on an insect, they begin to grow, burrowing themselves down into the creature's carapace.
Once inside, the fungus spreads throughout the insect's body, pushing its spear-like growths through the bug's inner tissues, eating the host from the inside. In most cases, when the host is dead and hollow - nothing but empty carapace - the parasite pushes its fruiting, tendril-like growths through the empty armor, erupting from the host's body. Spores then burst from the tips of the fungus, floating to a new host and beginning the cycle anew. The fungus can create impressive displays that look like multicolored clamshells, clusters of pink coral spines, cauliflower clubs and white corkscrews. They might almost seem beautiful if they weren't rising from the agonized cadaver of a spider or moth. Visually, the Infected owe a great deal to Cordyceps - the fungi clearly inspired the clickers' faces as well as the bodies plastered to walls, skin erupting with plate-like growths. It's something simultaneously attractive and repulsive that we've never seen before, making the infection the perfect candidate for a horror game. However, the real similarity to Cordyceps is how the fictional infection can take control of its host.
Naughty Dog based the infection on a species of fungus, Ophiocordyceps unilateralis, that has a unique ability to manipulate its host's behavior. After boring into a bullet ant, O. unilateralis infects the ant's brain, driving it to climb up a plant stem and clamp its mandibles onto the bottom of a leaf. Then, suspended 25 cm above ground, the fungus eats the ant from the inside out and, over the period of a few days, extends a single stalk out of the ant's head to release its spores. Science writers made the fungus notorious when they dubbed the manipulated hosts as "zombie ants," but that's not exactly accurate. Though scientists aren't yet sure how the fungus alters the ant's behavior, it's likely tricking the host rather than actually controlling it, leading to the more proper (but less sexy) term of "behavior manipulation."