Spoilers for The Last of Us.
CW: descriptions of fungus.
I was rather late to the party when it came to watching The Last of Us. I’m not someone who enjoys watching horror, but it was popular enough that I was convinced. Unsurprisingly, it was as good as everyone said it was, and somehow, a show full of undead zombies managed to pull off some stunning cinematography. However, I was surprised by the elegance of the cause of this zombie apocalypse.
If you’ve watched enough Planet Earth, you might recall the episode in which an ant, infected by fungus, becomes an insect ‘zombie’. This fungus is from the genus Cordyceps (and Ophiocordyceps), and it infects insects (such as ants) causing them to act strangely by ‘taking control’ of their bodies. It does this with the intention to spread it spores throughout the rest of the ant’s colony via its unwitting, possessed vessel.
Once the fungus has infected the ant it acts normally for a while. Then it becomes hyperactive, deviating from its usual instincts. Eventually the ant will climb up a plant and bite down on it, whereupon the fungus will use the host’s body as a source of energy, killing the host) in order to create a fruiting body (essentially a mushroom) which has spores that can infect more ants. The purpose of climbing upwards is to increase the chances of infections of other ants.
While the exact mechanisms Cordyceps uses to change its host’s behaviours are not known, scientists believe that it gains this control through chemical changes in brain of the insect, for example by altering dopamine and serotonin receptors to change behaviour.
In The Last of Us, Cordyceps has mutated due to climate change, and so it can withstand higher temperatures and infect mammals, including humans. It is estimated that “95 percent of fungal species…cannot survive at average human internal temperature” [1], which is why most human fungal infections are on the skin, rather than occurring internally. But there are concerns that rising global temperatures are causing fungi to adapt, which could be one of the factors causing the ‘recent surge in Candida auris cases across the globe’ (Candida auris is a very dangerous fungal infection, that kills a third of infected within a month). However, this isn’t currently a concern for Cordyceps, so we’re safe from that particular apocalypse.
But the most pressing issue we can take from the show, is realising how we’re so underprepared to deal with fungal infections. Most of the people who get serious fungal infections have weakened immune systems and are therefore not able to receive ‘live vaccines’ (vaccines that contain a weakened or dead form of the pathogen to trigger an immune response) as they can still react negatively to these; the other forms of vaccines that are safe for immunocompromised people tend to be less effective. It is estimated that “about 1.5 million people die a year as a result of [fungal] infections”, but some think the real number may be higher, due to underreporting of deaths caused by fungal infections in people with other major health problems.
While most fungal diseases are spread through contact with the environment, rather than from person to person, Candida auris is an exception to this rule. What makes it more deadly is its resistance to many of the few fungal treatments available, and its partial resistance to “disinfectants and heat…[making] it extremely difficult to eradicate”. Furthermore, fungi are “quite closely related to animals” so “any drugs that can interfere with a fungus’ growth and development are often quite toxic to us”. Despite this issue, and the WHO’s growing concerns about fungal infections, global funding for fungal disease research is very low and doctors are often undertrained in spotting such infections.
While fungi are not the most pressing health concern for humans, and a Cordyceps based apocalypse is very unlikely to happen any time soon, the threat of serious fungal infections is growing, and we should heed this warning, come as it may, from a TV show.
[1] https://www.wired.com/story/fungi-climate-change-medicine-health/
Bibliography
https://www.vox.com/culture/2023/1/21/23561106/last-of-us-fungus-cordyceps-zombie-infect-humans
https://theconversation.com/the-last-of-us-why-making-fungal-vaccines-is-so-challenging-201894
