One of the greatest examples of nature's cruelty you can find is the existence of parasites. Parasitism is a relationship between two species in which one benefits by harming the other. The one that is being harmed is usually referred to as the host. This is distinct from predation, wherein one species eats another as a food source. A predator will consume what it needs to and then moves on. A parasite will remain tied to its host as long as possible in order to exploit them.
Parasitic organisms can be found in all four biological kingdoms and live in every conceivable biome. They come in many different forms. There are parasitic birds, perhaps most famously the cuckoo, that sneak their eggs into other nests. The Last of Us was inspired by the real-life Cordyceps, a fungus that zombifies insects. Mistletoe survives by stealing nutrients from trees. Anglerfish developed a form of parasitic reproduction where males are permanently fused into the female's body.
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| A male anglerfish is permanently infused to a female's body, acting as a parasite by stealing nutrients from anything she eats. Original photo by Edith A. Widdler |
The abundance of parasites has even created its own ecological niche. Several species of fish effectively exploit the existence of parasites by acting as a sort of "cleaning service" that eats them off of other creatures' bodies.
The worst part is it's not even their fault. Parasites don't act maliciously or out of love for causing pain. They do it because natural selection has forced them onto an evolutionary path where they can't survive unless they parasitize. Because parasitism can, unfortunately, be a very effective survival strategy. So if there is alien life somewhere out there in the universe, whether intelligent or not, why shouldn't we expect to see examples of parasitism on other worlds and in alien ecosystems?
In honor of spooky season, we're going to dive into the disturbing world of alien parasites. I've collected the grossest, most terrifying parasites I could find from across film, TV, video games, and comics. And we are going to scientifically determine the deadliest parasite science fiction has to offer.
Note: This article was inspired by a piece I previously published on Game Rant. While some of the information from that article was reused, I have rewritten each entry to avoid self-plagiarism (that's apparently a thing) and tried to expand on the original concept, as well as provide updated information when available.
The Imposter- (Among Us)
While Among Us doesn't take itself particularly seriously, it does present a pretty unnerving setup. The cartoon aesthetic, silly cosmetics, and memes might take the edge off your fear, but how would you feel being trapped on a ship with the knowledge that one of the people around you isn't who they claimed to be?
The Imposter operates by infiltrating spaceships, consuming a crew member, and then assuming their identity. This gives them the ability to blend in, forcing the crew to use deductive reasoning to find them. When played effectively, the imposter can sabotage critical systems and eliminate crew members without drawing suspicion. A particularly crafty imposter can even divert suspicion onto other players, increasing the odds of an innocent person being thrown out the airlock.
Devastating as it is, however, the imposter only works at a small scale, and lacks the spreadability of many parasites. It can eliminate crew members but cannot infect anyone outside of its initial host. This makes it pretty tame in the grand scheme of things.
Fib- (Veggie Tales)
The Fib, also known by the scientific name Fibrilius Minimus, is a parasitic alien that takes a rather unconventional approach to its hosts. Instead of physically infecting them, it feeds off of dishonesty. When it lands in a heavily populated area, Fibrilius Minimus will scout the population in search of an individual who is in some sort of trouble, usually someone who has made a mistake or performed an immoral act and fears the consequences. This person will become their unwitting host. Children are an ideal target, due to their lack of critical thinking skills and ability to come up with lies that easily fall apart under scrutiny.
Upon making contact, the Fib will act as a friend while playing into their host's worries, convincing them it is better to lie about their misdeed than to admit fault. With each lie, Fibrilius Minimus is able to grow larger. Their growth rate accelerates with increased lies, so they will encourage the host to dig themselves deeper by continuing to lie even after being called out. In a matter of hours, Fibrilius Minimus can grow from a tiny ball-shaped alien to a godzilla-sized monster, at which point it will turn on and eat its host.
Admittedly, we don't have a full picture of the Fib's life cycle, so it's hard to say for sure how easily it spreads. However, individual instances don't seem to be contagious. They tend to focus on a single host and conveniently "disappear" in the presence of witnesses. That said, the final stage does make them capable of destroying a city, so we shouldn't underestimate them.
However, Fibrilius Minimus's method is also its greatest weakness. Just as it thrives on dishonesty, honesty can bring it down. The only known way to weaken or even kill it is for the original host to come clean about their lies. This is a pretty simplistic solution next to some on the list.
The primary method of travel used by Fibrilius Minimus appears to be via asteroids. The likely way it works is riding on asteroids until they enter the atmosphere of a habitable planet, then ejecting themselves into a populated area. While effective, it is not the most efficient means of spreading as it lacks any control of where it ends up.
Headcrabs (Half-Life)
Anyone familiar with the Half-Life games will be familiar with the infamous headcrabs, a parasitic organism that encases the head of its host. They've faced off against them and witnessed the horrific fates of their victims. The infamous cry of a headcrab zombie betrays the anguish experienced by those unfortunate enough to be hit by one.
Headcrabs have a simple way of functioning. Once they find a host, they jump up and encase their head (hence the name). They then embed their arms into the host's shoulders and seize control of their nervous system. The process results in the body being contorted and mutated, all while the victim is still alive and fully conscious. The only hope for release is death. It's a cruel fate, one weaponized by the Combine, who love to deal with suspected dissidents by launching a rocket full of headcrabs at them.
However, the very fact that they can be weaponized betrays a much darker truth. As horrific as they may be, headcrabs are actually on the weaker end as far as parasites go. Their only method of infection is to physically secure themself on the host's body, and it is possible (if difficult) to kill one before this happens. With sufficient precautions, they can be handled without issue. The Combine were able to use them easily, vortigaunts are able to consume them as food, and Kleiner even kept one as a pet simply by debeaking it.
While the results are nightmarish, Headcrabs ultimately lack the spreadability of many others, making them seem tame in comparison to others.
Seed Pods (Invasion of the Body Snatchers)
Seed Pods specialize in infiltration, which isn't unusual for a parasite, but they have one thing others don't- they don't have to physically infect their hosts. All that's needed is for a potential host to be in proximity to a seed pod. The pod produces a copy of the host. When the host falls asleep, they die and get replaced with the copy. That copy will then assume the host's identity while helping to continue spreading seed pods to infect more hosts.
What makes seed pods particularly dangerous is their discretion. They start small and gradually work their way out, carefully blending into the population and diverting any questions or investigation. Often it works so subtly that early signs are easily ignored. They come in the form of things like a kid saying there's something off about his mother, something easily disregarded by others. By the time they make themselves known, they're too late to stop.
Technically it is possible to spot a host- a general lack of emotion is a dead giveaway. However, by the time anyone can start identifying them they will have already spread to a point where they are really hard, if not impossible to avoid. The seed pods themselves can get so widespread that even the most resourceful survivors can be infected purely by chance. It was a total fluke of luck that any government response could be made to them, and that was the ending Don Siegel was forced to put in. His original vision was to end with the protagonist realizing how far it's spread and trying in vain to warn others (and the viewer) about what's coming.
The one limitation the seed pods have is they lack a means of self-spreading. While they can reproduce in alarmingly large numbers, actually spreading requires making use of their hosts and existing technology. Just covering the immediate area requires seed pods to actively be carried or loaded into vehicles to spread. Although this limitation does show them to be resourceful and adaptable to their environment, it does slow them down compared to others.
Genestealers (Warhammer 40k)
Tyranids usually function as predators, rather than parasites. However, there is one exception in one of their more... nefarious strategies. Genestealers are a special kind of Tyranid. They sneak aboard ships to get into worlds populated by other races. Once in place, they begin building up a cult, usually by seeking out outcasts and marginalized groups and slowly working their way into positions of power and influence. Over a few generations, their DNA will also start to infect the local population, producing hybrids. This makes them both a social and a biological parasite.
Left unchecked, a genestealer cult can gain enough power to dismantle a planet's defenses and destroy government infrastructures. Which is exactly the point. The goal, ultimately, is to leave the world vulnerable to a full-on tyranid invasion, essentially using parasitism to aid a predator. They can spread across an entire world without being noticed until it's too late.
The one silver lining, if it can be called that, is that genestealers are usually confined to a single planet. Once the final stage is complete, the genestealer and all its hybrid offspring are consumed as biomass along with everything else, rendering them incapable of spreading further.
Xenomorphs (Alien)
The titular aliens from Alien function as both parasites and predators. As predators, they are apex hunters. A single fully-grown xenomorph can decimate a group of humans in a matter of hours. A whole colony can be too much even for a group of heavily armed colonial marines. Xenomorphs are great at using their environments to ambush their prey. It doesn't help that they're intelligent and fully capable of strategizing.
As parasites, on the other hand, they have a messy reproductive cycle involving the infamous and aptly named facehugger and chestburster. The facehugger attaches itself to the host's head and shoves a tube down their throat through which it places the chestburster. Once that's fully grown, the chestburster will violently tear its way out of the host's body, killing them in the process.
As if that weren't bad enough, the facehugger has a cruel trick. It exploits the empathy of others to deter any effort at removal. It tightens its grip on the host's neck if any attempt its made at pulling it off. And the highly corrosive blood prevents it from being cut off without risking grievous harm to the host, the cutter, and any bystanders. Facehuggers can also burn through glass, so wearing a space suit won't offer any protection.
The good news is the facehugger alone isn't contagious. A single facehugger will attach itself to one host and die upon successfully implanting the embryo, so anyone around the host is unlikely to be infected. At least, that's if there's only one of them. While removing the facehugger is a huge gamble at best, it is at least theoretically possible to remove the chestburster via surgery if it is spotted early enough.
To really get going, xenomorphs need to mass produce eggs... which is exactly what they do. The crew of Nostromo found hundreds of eggs, but were fortunate enough to only see one actually hatch. The settlers at LV-426, fifty-seven years later, weren't so lucky. However, this requires the establishment of a queen and a sufficient supply of potential hosts. While effective, this does take time and requires xenomorphs to jump through a few hoops before they can reach the level of other parasites.
Zerg (Starcraft)
The Zerg are driven by an endless quest to achieve genetic perfection. To that end, they have an unusual biological quirk- an ability to consume other organisms and absorb their genes. This ability makes them highly adaptable to different environments, including outer space. By taking traits from other species, Zerg can produce a variety of different units for different tasks. There are even zerg who have evolved to act as living spaceships, facilitating interstellar travel.
Naturally, they spread very easily and can consume entire worlds given the opportunity. While it is possible to resist them, this is easier said than done. They are extremely durable and resistant to most conventional weapons, and their sheer numbers can be overwhelming. There's a reason they are often referred to as a swarm.
Luckily, there is one thing that's stopped them from consuming the galaxy outright. They are pragmatic and can be reasoned with. Under the right circumstances, they can form alliances and make an uneasy truce with other races. And even uphold that truce. They were even open-minded enough to make a human their queen. While certainly not an easy thing to broker, it is more than can be said for a lot of parasites.
Mnggall-Mnggal (Star Wars)
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| Note: This may or may not be an actual image of Mngall-Mnggnal. It's been alternately labelled as Mgnall-Mnggal and a DND monster named Jubilex, and I have been unable to find the picture's original source, so I can't confirm which it is. |
What I said earlier about parasites not being to blame for their actions? Yeah, that all goes out the window with Mnggal-Mnggal- a parasitic creature so twisted and evil, you could probably be forgiven for thinking this was Warhammer 40k's answer to the Thing instead of something out of Star Wars. What makes mnggall-mnggal particularly sick is its malevolence. It doesn't just parasitize to survive; it revels in it. It loves tormenting its hosts and using them to play mind games.
Mnggal-mnggal is essentially a sentient fluid, which makes it easy to infect a potential host. It commonly appears as a seemingly inert pool of black goo, waiting for a potential host. When one appears, it will splash the unfortunate victim, quickly sliding into any opening it can find in the body. A single drop will be more than enough to be lethal. mnggal-mnggalwill begin eating the host's body from the inside out, breaking down organs, tissue, and bone, until only the outer flesh remains. It will then continue to puppet its host until their body inevitably decays.
This does make mnggal-mnggal easy to spot if one knows where to look. Zombified hosts have several obvious signs. However, what makes mnggal-mnggal dangerous is it's intelligence and desire for cruelty. It won't just infect anything in sight, it will actively communicate with them. mnggal-mnggal will offer lost secrets and forbidden knowledge that it may or may not actually have. One of its favorite tactics is to possess children and then sic their zombified remains on their parents. Some go as far as to suggest it doesn't even need to feed on its hosts and acts purely for fun. It even collects trophies in the form of shipwrecks orbiting its apparent homeworld of Mugg Fallow.
Judging by the state of their homeworld, which is completely taken over with mnggal-mnggal having destroyed just about everything, it can easily take over an entire planet. And has probably done this across several worlds. It also has a very simple mechanism for spreading. Once a host is taken, their remains can easily be used as a vector for spreading the goo to others. Gnall-Gnall can also use its hosts to pilot ships, so spreading to other worlds isn't a problem either.
An important detail to keep in mind is the mnggal-mnggal isn't a species in the traditional sense. It's actually more like a single organism. Every instance ultimately connects to the same hive mind, making it great at co-ordination to the point where zombified hosts will often act in unison.
Luckily, it is possible, if difficult, to quarantine mnggal-mnggal. It's bad enough that the Chiss are willing to pay handsomely for anyone open to purging an infected world. Full eradication is possible in theory, but nobody's come up with a plan for how to safely approach Mugg Fallow and destroy it at the source. And even then it's hard to say if that would be enough to destroy mnggal-mnggal for good.
The Thing (The Thing)
The titular monster of John Carpenter's 1982 sci-fi horror classic The Thing was described as "the ultimate in alien terror" for a reason. Xenomorph infections are bad enough, but at least their victims can be identified with some level of reliability. Actually, dealing with the infection is another matter, but usually the presence of a facehugger makes it easy to tell who was infected. Not so much the case for The Thing.
The Thing is an alien that not only infects and consumes its hosts, but also replaces their bodies with an identical copy. There are a few ways it can do this, but ultimately, its greatest strength is discretion. The ability to perfectly copy its host allows the Thing to easily blend into any environment and spread unnoticed.
Once it gets going, the Thing is really hard to eradicate, because all it needs is a single cell. While an individual instance of the Thing can be defeated by a flamethrower or explosives, it remains a threat so long as any cellular activity remains. And even when one instance is dealt with, its rapid spread and built-in survival instincts make it easy for smaller pieces to break off and escape.
But what makes the Thing especially dangerous is its intelligence. It doesn't just spread to whoever's nearby; it studies its targets and strategically decides who to infect. This intelligence can be used to exploit the inevitable paranoia of its victims and manipulate them against one another. It even has the knowledge to build its own transportation using whatever bits and pieces of machinery are available.
Left unchecked, the Thing could easily assimilate every organic lifeform on Earth in a few days at the most, and that's just the latest in what is probably a long chain of planets. Given sufficient means of travel, the Thing could spread across the galaxy and maybe even further.
X Parasite (Metroid)
Metroid's X Parasite is a gelatinous organism that consumes its hosts and replaces them with an identical copy, similar to the Thing. In their base form, X Parasites look like large blobs, usually yellow though variations in other colors exist. In this state, they are really hard to kill, simply because it's hard to actually do any meaningful damage. But it gets worse when they actually infect a host.
As a gelatinous lifeform, the X Parasite can easily squirm into the cracks of any armor and absorb itself into the host's body. They seize control of the host's nervous system and then begin reproducing. Each copy of the X Parasite begins taking over more and more of the victim's body, absorbing their DNA. When the process is finished, the host is replaced with a nearly identical copy. It will look like the original host, while also inheriting all their skills and memories. Just about everything short of emotions.
Unsurprisingly, the X Parasite can spread really fast, especially if it gets the right host. The entire infection process only takes a few seconds. For the Chozo, it got so bad that they had to genetically engineer a predatory organism to keep it at bay- this is literally the entire reason Metroids exist. And even they couldn't completely wipe it out.
Voidworms (Stellaris)
If some of the parasites above seem frightening, how about one that consumes entire planets? While many of the previous entries, like the Thing or Seed Pods, and the Genestealers, could easily take over a planet given the chance, they still have to infect on an individual level. That usually means gradually infecting hosts one at a time. World domination doesn't happen straight away, and there is at least theoretically time to identify and stop them. That all goes out the window for voidworms.
These space leeches travel across the galaxy looking for planets occupied by biological life forms. When they find a suitable world, they bombard it with spores, quickly overwhelming and easily infecting any living organisms on the surface, whose bodies become incubators for the worm's young. Eventually, they emerge from their hosts as nymphs and leave the planet, continuing the cycle.
While they don't appear to have the intelligence of other species, their ability to mass infect an entire planet gives them a firm reproductive advantage. It is possible to develop a cure to the infection and even an immunity, but this takes time- easily enough for the infection to spread before a cure can get anywhere near completion.
Another thing that makes voidworms especially dangerous- they can travel through the vacuum of space. Keep in mind that other examples, like the Thing or even mnggal-mnggal, still need to either build their own transport or exploit their host's technology. Voidworms can travel through space unimpeded and, while not indestructible, become formidable opponents when allowed to grow to their full size.
The Flood (Halo)
That alone would be bad enough, but it gets worse. The flood considers intelligent species a primary target because it can learn from each host's mind. When a host is infected, the flood gets access to their mind. It can read their memories and access everything they know. The more intelligent hosts consumed, the more it learns. This allows the flood to become a lot more organized and strategic. Once it starts forming a gravemind, the Flood can start using advanced battle tactics and even play conflicting sides against one another for its own ends.
Brethren Moons (Dead Space)
The Brethren Moons are massive aliens, similar in size to the Jovian moons, who have built their entire existence around exploiting the evolutionary patterns of other species. Their method relies on what could be described as a galactic trojan horse, known as the marker. It's like a cruel inversion of the monolith from 2001: A Space Odyssey. Where the monolith acts to further the evolution of other species into an intelligent spacefaring civilization, the marker presents itself to an established spacefaring species and engineers their downfall.
The whole system relies on a common evolutionary pitfall. Each species encounters a problem when it exhausts their homeworld's resources and must travel into space to sustain itself. The Brethren moons offer the marker as an apparent solution- a free and infinite power source, and who's going to say no to that? Even better, they're designed to be easily studied and reproduced. The host species, seeing a perfect solution to their energy problems, will begin making more markers and dispersing them across every planet they've settled. All while failing to realize this is only step one of a bigger, much darker plan.
By becoming an integral part of the host species' society, the markers are able to secretly begin manipulating people around them with electromagnetic frequencies. This begins discreetly, with the marker causing people in proximity to begin experiencing hallucinations. This is used to create chaos which then leaves bodies that can be reanimated as necromorphs, corpses modified into durable killing machines that further the infection. The final stage is a convergence event where all the leftover biomass is brought together and forms into a new Moon.
The biggest advantage the moons have is their ability to mass infect on a level unseen in most parasites, without even touching their hosts. While they are certainly capable of surviving in a vacuum, they don't even need to travel. The use of markers allows them to quickly infect multiple worlds simultaneously, while also taking their host species by surprise. It's already known that they previously destroyed thousands, if not millions, of other civilizations through this process. And each cycle only produces a new moon, adding to their numbers and making them increasingly powerful.
The only limitation the moons have appears to be a technological one. They can launch markers around the galaxy, but lack the means to actually build them, instead relying on their host species to replicate and spread them across any given civilization. But being as immensely powerful as they are, this is a pretty minor drawback.
Starro (DC Comics)
Now, admittedly, when it comes to this one, the available data can be wildly inconsistent, thanks to DC's tendency to constantly rewrite its own canon. Is he a single entity or part of a larger species? What exactly is the full range of his powers? Is he even truly a villain or more of an anti-hero? It all depends on exactly which version we're looking at across a huge range of comics from different eras, plus movie and TV versions.
That said, Starro is usually portrayed as a ruthless tyrant seeking power and control. To that end, he relies on a seemingly infinite supply of parasitic starfish that can be launched at will. These will attach themselves to a host's face and override their nervous system. The host's free will is suppressed, and their body effectively becomes a drone for Starro's bidding.
Whether the host can survive this process depends on the version, but Starro can launch thousands of stars at once, and co-ordinate hosts across massive distances unthinkable to most parasites. At his most powerful, Starro has been able to infect and control hosts across nine galaxies. And he has even shown a capability to infect superheroes, having used his mind-control stars on several Justice League members on multiple occasions.
The Hiss (Control)
The Hiss are an extra-dimensional entity that even managed to overwhelm an entire US government agency dedicated to protecting extra-dimensional threats. The Hiss can infect and assimilate its hosts in an instant, transforming them into tools that further its spread. There are a few different ways hosts can be used, but they all serve the same function. At best, hosts get turned into transmission vectors who constantly perform a mysterious incantation that seems to facilitate their spread. At worst, hosts can turned into weapons, their skills turned on their former allies. Or even worse, the hiss can modify their hosts on a molecular level, twisting and contorting their bodies to whatever serves their purpose.
Some of this might not sound that unusual, but how many parasites can travel across dimensions and spread by literally breaking reality? The Hiss are powerful enough to easily overwhelm and confuse an entire government wing dedicated to dealing with extra-dimensional threats, taking the majority of its personnel almost immediately. Just Containing the hiss was only made possible by intervention from another equally powerful entity, and even that took massive foresight and extensive research from their head scientist. If not for Polaris, the Hiss would easily have overtaken the world and probably continued spreading, maybe even destroying the universe as they have in other realities.
No other entity on this list has the power to deal the amount of damage the Hiss can do when allowed to run wild. This easily makes them the deadliest parasite there is.
