In HBO's The Last of Us, a post-apocalyptic world is depicted where a fungus called Cordyceps Brain Infection (CBI) has turned most of mankind into zombies. Viewers may wonder if the "zombie fungus" is real and whether it poses a threat to humans. While the show is a work of science fiction based on a video game, the fungus featured in it is not entirely fictional.

The Last of Us is reportedly inspired by the Cordyceps fungus, specifically Ophiocordyceps unilateralis, which is known as the "zombie fungus" that infects ants. Though the fungus is real, experts say that it cannot infect humans. However, there are still questions surrounding the fungus. Here's everything you need to know.

What is Cordyceps fungus? Ophiocordyceps unilateralis is a specialized parasite that infects, manipulates, and kills ants in tropical settings, according to a scientific paper. Raymond J. St. Leger, a mycologist and professor in the department of entomology at the University of Maryland, who has worked on Ophiocordyceps unilateralis, explains that the fungus spreads through spores. When a spore lands on an ant, it produces a tube that bores through the ant's cuticle into its body. Once inside the ant's "blood," called hemolymph, the fungus multiplies and spreads throughout the body. St. Leger adds that the fungus grows around the ant's brain, causing the ant to develop specific behavior during the zombie stage.

How does Cordyceps fungus spread? The fungus spreads around ants in a specific way. Once an ant is infected, it crawls up a plant stem and bites down hard onto a leaf's vein with its mandibles. St. Leger explains that this "death grip" is caused by the fungus colonizing the mandibles. The fungus also comes out through the ant's legs as tubes that stick the ant to the leaf. The ant eventually dies, and the fungus produces a fruiting body from its head. Pereira notes that even if the ant doesn't spread spores over other ants, the spores can still spread by wind and water and eventually land on a new healthy host. Williams adds that the ant attaches itself to a specific spot on the underside of a leaf near where other ants are foraging, increasing the chance of infection.

What are the signs an ant has Cordyceps? An infected ant initially starts trembling and cleaning itself, but only very specific scientists might notice, according to Williams. Other ants recognize this behavior as a danger and will carry the infected ant away from the nest and dump it, according to St. Leger.

Can Cordyceps infect humans? While some pathogens do jump from animals and bugs to humans, experts stress that humans do not need to worry about a The Last of Us situation happening anytime soon. "It's really specific to a small group of ants," says Williams. "It's co-evolved with these ants to have a specific reproductive strategy." According to Thomas Russo, M.D., chief of infectious diseases at the University at Buffalo in New York, only a tiny fraction of microbes infect humans, and fungi have an even smaller number. While it's possible for humans to have an allergic reaction to the fungus, Pereira notes that it's unlikely to cause a true infection.