It's one of the most popular scenarios in modern horror: the deadly virus (preferably of the zombie/flesh-eating type, but we're not too fussy). It's been used countless times in cinema, from to Cabin Fever to Contagion and is a staple of pretty much every zombie movie to come out of Hollywood since Night of the Living Dead sparked off mainstream interest in the zombie film back in the 1960s.
But it's not just Hollywood that has drawn from nature's viruses for its terrifying themes, the zombie/virus danger is prevalent in video games too, from Resident Evil to Dead Rising. But some of the viruses portrayed in these popular games do in fact have a basis in reality, which makes them even more terrifying than you may have realized...
The Last of Us: Cordyceps Brain Infection (CBI)
Skimming over what a wonderfully crafted game The Last of Us is in general (and I could wax lyrical on that note) let's focus on their specific brand of zombie. Namely these guys:
In The Game...
The Cordyceps Brain Infection (CBI) is the cause of all those gross, fungal looking creatures that are constantly causing bother for Joel, Ellie and the rest of humanity throughout The Last of Us. If you've played the game you'll be well aware of the different types of infected you encounter; Runners, Stalkers, Clickers and Bloaters.
Each type represents a different stage of fungal infection wherein Runners are the weakest (and most human in appearance) and Bloaters are the most dangerous (and most mutated looking).
Spread through inhaling spores, the cordyceps fungus grows within the host body, gradually shutting down higher brain functions like logical thought, reasoning and morality. As the host brain shuts down they become increasingly aggressive and volatile and eventually lose their sight as fungus grows over their face and their visual cortex shuts down. They develop a form of echolocation (see Clickers) and if allowed to remain alive for more than a decade or so they eventually reach the final stage, the host body taken over completely by the cordyceps becomes a Bloater.
The bad news is that cordyceps IS a genus of fungi; it DOES exist within nature and it IS deadly. In fact there's around 400 species of cordyceps that we know of right now, some are even used as medicinal mushrooms in traditional Chinese medicine.
Take for example cordyceps sinensis, a species of cordyceps fungi which infects and parasitises the bodies of ghost moth larvae (having a cool name wont save you this time ghost moths).
The fungus spores infect the living larvae before killing and mummifying the teeny tiny body, from which it begins to grow outwards in an ascocarp (fruit body) - a stalk like protrusion which releases further spores into the environment. And the circle of life continues.
That's the parasitic basis of cordyceps: to infect, kill and replace the tissue of the infected host. That's the bad news. The good news is that the fungus only affects insects and other arthropods.
The cordyceps in The Last of Us is based on a fictional mutated strain of the fungus, not one that is found in reality. But you know, fungi is constantly evolving...
The really freaky part though? Like in The Last of Us some known cordyceps species are able to affect the behavior of their host, such as ophiocordyceps unilateralis which compels infected ants to climb to the top of a plant and attach themselves there in order to achieve maximum spore distribution once they die.
Woah, looks like you lose this round ghost moths.
Left 4 Dead: Spanish Influenza/Rabies
The Left 4 Dead zombies borrow more from classical zombie mythology than those of The Last of Us, but there's a viral outbreak source here too in the form of the Green Flu.
In The Game...
The Green Flu is an infection of unknown origins transmitted virally from person to person, turning them into the aggressive zombie-like "Infected".
The Left 4 Dead Infected also have designated zombie types; the Common Infected, who you encounter constantly throughout the games, the Uncommon Infected who are a tad more problematic and the Special Infected who are the worst of all. The Uncommon Infected include the CEDA Worker, Jimmy Gibbs Junior, the Fallen Survivor, the Clown, the Mud Men, the Worker and the Riot.
The Special Infected you'll be well versed with if you've played the game. They're stronger, smarter and more heavily mutated than their Infected brethren, each with a unique ability and attack style. The Special Infected are comprised of the Smoker, Boomer, Hunter, Jockey, Spitter, Charger, Witch and the dreaded Tank.
In Left 4 Dead it emerges that the CEDA designated the unknown viral strain as a form of influenza, though it's becomes likely that this is nothing more than a cover up in a failed attempt to curb initial panic when the outbreak first began. But influenza isn't that deadly, it's just the flu - experienced by most of us at some point in our lives - right?
Okay so to be fair the influence of the Spanish Influenza on popular zombie shooter series Left 4 Dead is based more on the social and governmental effects the outbreak had on the populace, at least according to Valve's Chet Faliszek, the lead writer on Left 4 Dead 2.
"We’ve done a lot of research on things like the Spanish influenza and that’s a good example of a major epidemic spreading, and in the Sixties there was a flu that spread and even swine flu."
But what was the Spanish Influenza? It resulted in more than 50 million deaths and caused widespread panic when it swept the globe back in 1918 and was a particularly deadly strain of influenza.
In comparison to typical influenza, which usually kills the very young, very old or very sick, the Spanish influenza took out mainly healthy young adults, making it as unavoidable as it was terrifying.
We're still not 100% sure why this happened, but contemporary research has suggested that the Spanish influenzal strain killed by causing an overreaction in the host body's immune system (called a "cytokine storm"). Stronger immune responses like those of healthy young adults caused greater damage, whereas those with weakened immune systems were more likely to survive the virus.
It's not difficult to see the parallels between the Spanish influenza strain and the Green Flu here - a virus that targets and spreads through the strong. And in Left 4 Dead the Green Flu does spread like the flu it's named for, the only resistance against it is to be born with a recessive X chromosome passed down from father to child, there's no known vaccination or cure for it.
However it's been heavily suggested throughout the games that whilst the Green Flu may have spread like influenza it actually is more likely to be a mutated strain of the rabies virus.
Rabies sounds scary, but it's only really dangerous in animals: a neuro-invasive disease which causes brain inflammation and strange, often aggressive behavior. Aggression isn't a typical sign in humans though, more likely for people infected with rabies is symptoms of fever, headaches and anxiety.
However, imagine if you had a mutated strain of rabies spreading with the ferocity of Spanish Influenza... In fact let's not think too hard on that one.
Resident Evil IV: The Plaga
Resident Evil popularized and redefined the zombie shooter game, so it's no surprise that they made the list, but did you know that the behavior altering brain parasites encountered by Leon Kennedy in Resident Evil IV are based on a strain from reality?
In The Game...
The Plaga (or Las Plagas) are a type of infectious parasite used by the cult who kidnapped the President's daughter Ashley in Resident Evil IV - Los Illuminados (The Enlightened Ones).
The parasitic Plaga attaches itself to the nervous system of the host body, sheltering between the lungs in order to access the spinal cord and affect the host's behavior. However, rather than destroying the host body, the Plaga controls and utilizes it socially via a hive-mind of sorts.
As the Plaga grows larger inside, the host symptoms start to appear; the coughing up of blood, bleeding eyes, convulsions, hallucinations, fainting spells, swollen and discolored blood vessels and red glowing irises.
The parasites can only be removed via targeted radiation therapy, and only in very early stages of infection. Once the Plaga has attached to the host's central nervous system it's nearly impossible to kill them off without killing the host. The further through the Plaga stages the parasite grows, the more deformed the host becomes...
The Enlightened Ones intended to use the President's daughter, Ashley Graham, as a host for the parasite in order to infect her father. With the US President under the command of the Plaga, and in turn the Enlightened Ones, they'd be able to achieve their goal for world control. The cult members are all willingly infected with Plaga too because why the hell not tick all the boxes on crazy person bingo?
Ok, so mind controlling death parasites sound pretty scary, but they can't be real, right?
Well... There is this little thing called toxoplasma gondii, a protozoa which has been observed to have the ability to alter the behavior of infected rodents. Toxoplasma gondii affects the rodent's behavior with a view to make them less likely to avoid or run from predators; namely cats.
Because cats are the only hosts in which toxoplasma gondii is known to be able to reproduce within, the manipulation of the rodents behavior is speculated to occur as a stage of an evolutionary adaptation to increase the reproductive success of the parasite. Well that's creepy.
Okay, but that's just in rodents right? Well, toxoplasma gondii is able to infect pretty much all warm-blooded animals, including humans. In fact, it's one of the most common parasites found in humans, there's about a 50% chance that you're infected with one right now.
Don't panic though! It doesn't necessarily mean that you're gonna go full zombie anytime soon (never go full zombie), but there has been observed reports of toxoplasma gondii altering human behavior, though not quite to the extent that it does in rodents. For example, those infected are thought to be more prone to schizophrenia, a severe and chronic brain disorder.
But perhaps the scariest part is the studies that show toxoplasma gondii infected males seeming to demonstrate a lower super-ego response. The super-ego is the part of the psychological personality system that controls the impulsive, animalistic id and ego by imposing moral boundaries. Imagine what happens to a mind if that's taken away?
Of course none of this is definitively conclusive, and the fact that 3 billion of us aren't shuffling around like schizophrenic zombies is certainly comforting, but what happens if the parasite evolves...?
I guess it's a good thing there aren't any giant cats hanging around.