Cordyceps is a fungal pathogen that attacks insects, using them as food for growing fungal stalks and spreading the fungal spores over a region, thus, infecting multiple hosts. In its most dramatic, and probably best-known, form, it literally manipulates an ants behavior. The ant will climb up into the area above the soil, where Cordyceps is happy and has the best growing conditions, and gradually a fungal body emerges while the ant is trapped and dying. If an ant is stunned early, ants quickly scoop it up and carry it away to keep the Cordyceps from wiping out the whole colony.
Cordyceps and The Last of Us
Naughty Dog has been going after more of a movie-like game style for years, and one of the many things that has picked them up is taking a shot at scientific nonsense. Not a rare occurrence, but I have had a few people tell me that they are concerned Cordyceps might actually infect humans, and such misinformation is one that I wanted to rectify, by breaking down what Naughty Dog has created, and explaining why an infection with Cordyceps in humans similar to the version in The Last of Us is extremely unlikely. Also, in the interest of full disclosure, I did not play the game or view any of its cinematics, yada, yada. I am going to be reading about some story beats, as well as the games explanation for infection, in order to go against that, but I felt it was something that I needed to point out, though the main focus of this blog has very little to do with the games story itself, and much more with the most visually dramatic parts.
What is Cordyceps?
Your first thought when you picture mushrooms is likely to be a classic fungus. Unlike most microbes, which can rarely form a big body, many fungi species do form big structures. These structures grow in order to enable the spread of spores over large areas, which allows the formation of new mushrooms and continues fungus lineages.
To understand why it is highly unlikely that cordyceps would infect humans, first we must understand what the fungus is in this context, and what cordyceps is in and of itself. I am going to be using Wikipedia for some of this information, and I will reference it if I find any other good sources, but generally, Wikipedia offers a nice fusion of information, and I feel confident enough with this, I do have a fair amount of knowledge about fungi, but not anything specifically about this fungus, so the request.
With fungi, you have two types of growth-hyphals, consisting of stringy cells, and yeast, consisting of larger, individual cells. Cordyceps is, like most real pathogens, a dimorphic fungus, that is, it has a yeast phase as well as a hyphal phase.
First, the spore falls onto an ant, forces its way through the ants body via enzymes, and then it propagates via a yeast-like form. When the time comes to create the fruiting body, spores create a fungus structure, it becomes mycelial and grows into a string, slowly gobbling up the now-dead ant. Eventually, a fruiting body bursts out of the ant, grows until it is just right, then releases spores into the air, looking for a new host to land on.
Clearly, in a way, it is a winning strategy, considering many different insects have their own tiny, nightmarish mushroom to contend with, it is the dramatic process that grabs the imagination, and the allure of this to various media is entirely understandable. Now you know what it is, I can a bit better explain why it is unlikely to be able to infect humans.
Behavior Modification in Insects, not Humans
There is a world of difference between the human brain/nervous system and what insects have, a gross oversimplification, but a significant one. An insects brain and nervous system is nowhere near as complicated as a humans version–meaning trying to bend it to a pathogens whim is easier on a bug than on a person.
Behavior modification is one thing for pathogens, of course, but cordyceps infects insects alone-shifting from insect to human neural systems and brains would be almost impossible without hundreds of years of evolution, and even then, probably not in their favor, with insects being abundant and fast-breeding, whereas humans are less so. Also, Cordyceps doesnat send a strike signal into the brains of the ants-it sends a locational signal, so that the ants end up in a place that Cordyceps is happier. It then makes the ant cling onto the leaf and stay there dying, as Cordyceps consumes it.
So, there is not really any reason for the thoughtless aggression, presuming that a cordyceps can infect humans, chances are that it will be trying harder to get them into a specific height and temperature zone so that it can burst open and disperse multiple spores into the air. This makes zombie outbreaks very unlikely-ignoring of course disease protections and diagnostics that we have that would put an immediate end to any zombie outbreak.
Cordyceps Method of pathogenic attack in The Last of Us
No real pathogenic fungus has ever descended upon the human skin to feed on its way inside, so Cordyceps has already fallen down that first hurdle. The interesting thing about human skin is that it is dead, full of all sorts of unpleasant methods for killing off pathogens-and also has its own indigenous flora, which makes it that much more difficult to grow.
Traumatic implanting is one thing-but that is when you puncture your finger through a rose, and fungus grows out, so biting does not count. Also, the fungal pathogen is probably going to only infect your blood and lungs-not your saliva. And when compared with other fungal infections, usually the real pathogens are spread by spores that are inhaled-not by biting. If cordyceps were to infect humans, it would appear to use methods that are different from those used by true pathogenic fungi. That means that it would only spread by inhaling spores, it is not exactly the lowest-infection-rate kind of spread, but it does poke yet another hole in the zombie narrative.
Immune System Response
Another obstacle is the way human and insect immune systems are different. Insects have an innate immune response, basically, an unspecific system for attacking pathogens. We also have innate immune responses, however, we also have adaptive immunity, which is the beast of a defense system, which has multiple steps.
Most actual fungal pathogens, and fungal pathogens generally, do not tend to get into people who have a healthy immune system-hence for a long time, anti-fungal drugs were a bit restricted and uncommon. Also, our blood circulates differently from that of insects, having closed systems rather than open circulatory systems of insects. In addition, we have an extensive arsenal of antifungal drugs available now, so it is highly unlikely that any newly infectious organism will have an easy time battling its way to the infective population.
Also, in-game, as far as I can tell, Ellie has an equable fungus form on her brain. Because of the fungus being big and growing on the things that she is eating, it is very possible she will get brain damage or inflammation from the fungus because of the breakdown in the brains blood-barrier by her immune system, the kind of infection that will probably kill you in the medically-depleted apocalypse. A proper immune response would have been a much more likely occurrence-where, because of her genetics, she managed to have an immune reaction that killed off Cordyceps and prevented her from becoming susceptible. A milder strain of fungal pathogen makes less sense, particularly given her position.
Evolution is a powerful tool that allows infectious agents to disperse and adapt to their hosts, they sometimes can even disperse between species. But in order for a fungal strain like cordyceps to spread to humans, it had to go through an absurd number of changes in its genome. In The Last of Us, a virus gets into the funguss genome and causes it to leap into humans. Unless it is the largest virus to insert itself into the genome on Earth, which is beyond the capabilities of any virus we have seen to date, there is zero chance that this kind of thing could happen.
The viruses are small, and I doubt that any gene reassortment would have made this into the ideal human pathogen all at once. With each hurdle, entry methods, more sophisticated immune systems, different types of cover, and advanced operations in the human brain, Cordyceps will have to fit all those things to be infected in humans. And when it was able to do so, who is to say that it would do the same for humans, by the time it became a human pathogen, it probably was much closer resembling other human fungal pathogens than to an insect pathogen. Without humans living in continuous contact with cordyceps, and given how specialized fungi are, I do not see much evolutionary incentive for a fungus to infect humans. And even then, it would be hundreds, perhaps thousands, of years before it reached the stage that could do the sort of damage that the fungus has done in the most recent of us.