Nature can be cruel and one need look no further for proof of that than the existence of parasites. In its simplest definition, parasitism is a relationship between two species in which one partner benefits by harming the other. Parasites are everywhere. They can be found in all four biological kingdoms and come in many different shapes and sizes. Parasites are so common that some fish evolved to provide a “cleaning service” for other species by eating parasites off of them. Anglerfish have a unique form of reproduction in which males parasitize females by becoming permanently fused to their bodies. It has even affected language, as people will often use the term “parasite” to describe an individual who lives at another’s expense without offering anything in return. The worst part is it’s not even their fault. Parasites literally evolved to a point where they have to parasitize in order to survive.

Parasites can be terrifying. What else can be terrifying? Outer space. The possibility of life on other planets has long fascinated humans, but it has also terrified them. After all, there is no way to know what it might look like until it is discovered. And if parasitism is such a common survival tactic on Earth, is it really a stretch to imagine that an extra-terrestrial species could adopt the same strategy? It is hardly any wonder than that human imagination has conjured up a variety of disturbing concepts of alien parasites.

12 The Imposter (Among Us)

An Alien Hidden in Plain Sight

If we put aside the cartoonish aesthetic, the silly cosmetics, and the memes, this is actually a pretty terrifying setup. Astronauts are stuck on a spaceship or station with an unknown alien that hides among them (hence the title). While it is unclear how this alien infiltrated them in the first place, its unique talents pose a serious danger to the crew. Obviously, it can impersonate any of the astronauts, not only replicating their bodies but also their spacesuits. The impostor is also intelligent and tactically-minded. It can discreetly sabotage important systems, quietly eliminate crew members, and throw suspicion on others. It also has the unique advantage of being able to get into places that astronauts cannot, which is great for moving around.

The biggest advantage the Impostor has is its ability to work quietly without drawing attention. However, the main challenge is identifying the impostor. This part can be difficult, but when it is found, it is pretty easy to defeat. Throwing it out of the airlock is all that is required for the danger to be removed, and it appears to leave no traces that could potentially cause further harm. The Impostor also lacks an ability to take on more than one host, and therefore is not easily spread.

11 Headcrabs (Half-Life)

Keep Them Off Your Head

As the name implies, headcrabs are creatures that function by fixing themselves to the heads of other species. The results are unsettling, to say the least. Upon attaching, the headcrab fixes itself to the body using its claws, then takes control of the host’s nervous system. After a period of time, the infected body begins to mutate, developing claws where the hands would be and exposing internal organs, with more extreme cases even losing skin. The worst part is not only is the host still alive, but they are also fully conscious during this process. They can be heard screaming and crying for help. Half-Life 2 makes it clear that death is merciful next to getting headcrabbed, to the point where Father Grigori has dedicated himself to mercy killing the infected population of Ravenholm.

That said, as terrifying as the process is, headcrabs are actually on the weaker side of alien parasites. It does not take much to kill a headcrab. As long as it can be spotted before actually attacking someone, it can be defeated with a simple crowbar. They can be a problem in larger groups, especially when weaponized. Launching a pod full of headcrabs into a populated area can have devastating results. However, the very fact that they can be weaponized speaks for their relative weakness compared to other entries on this list.

10 Illithid (Dungeons and Dragons)

Brain-Eating Monsters that Can Dominate Minds

More commonly known as “Mind flayers,” these infamous creatures are rightfully feared by many. The very idea of having one’s brain consumed while they are still alive is discomforting enough, but that is barely scratching the surface. They also have a few different uses for parasitism. Baldur’s Gate 3 players will already be familiar with their method of reproduction. Illithids begin life as leech-like “tadpoles” that get inserted into a victim through their eye. The tadpole then squirms its way into its brain and starts the agonizing process of Ceremorphosis, which modifies the body until it is ready to transform into a new Mind Flayer. However, parasitism is also used as a means of control. Mind flayers can use tadpoles, and their psionic powers, to subjugate other races. By dominating their minds, individuals are turned into slaves with no free will, sometimes without even realizing it. To make things worse, Illithids are also super intelligent and technologically advanced, capable of traveling across dimensions. At one point they even ran an empire, which they will stop at nothing to restore.

Illithids certainly live up to their reputation, though their method of spreading is a bit more restrictive. It requires capturing potential hosts and putting the tadpole in them. They are also more selective in terms of when and who they infect. Reproduction is not always a priority and when they infect a host, it is more likely to be a strategic decision or a means to an end rather than purely reproductive. They can perform mass infections like they do in Baldur’s Gate 3, but this is normally only done if there is something to be gained from it. Combined with the rarity of actually encountering one, the chances of an infection, let alone it spreading, are actually pretty low.

9 Genestealer (Warhammer 40k)

Slow Methodical Erosion of a Planet’s Society

Tyranids are usually predators who specialize in consuming life anywhere they find it, but they do have one particularly cruel tactic that involves turning to parasitism. Enter the genestealer, a unique form of tyranid that sneaks onto populated worlds (usually by hiding in ships) with the ultimate goal of making them vulnerable to invasion. This is usually accomplished by starting a cult, appropriate since a lot of cults act as parasites in a more figurative sense. However, there is also a literal aspect in this case. Aside from using the BITE model, genestealers insert their DNA into their followers, allowing subsequent generations to be born as hybrids. Left unchecked, a genestealer cult can reach a point where it is able to undermine governments and sabotage military resources undetected. All this sets up the final stage where the Tyranid fleet arrives and consumes the now-vulnerable planet.

Normally, what makes tyranids so frightening is their ability to swarm in overwhelming numbers and adapt to anything in their path. Genestealers are the opposite. They work from the shadows and meticulously dismantle their enemies’ defenses one piece at a time. And they can do it all without being noticed. The fact that tyranids can use such a tactic speaks a lot of their hidden intelligence. However, genestealers tend to be limited to one planet and are unable to spread further. This is because they are actually consumed along with everything else when the Tyranid Hive Fleet arrives, putting a distinct cap on their spreadability.

8 X-Parasite (Metroid)

Gelatinous Creatures That Take the Shape of their Host

If Metroids seemed bad enough, they pale in comparison to the creatures they were created to hunt. The X-Parasite is a gelatinous organism that infects its prey and slowly consumes them from within. When the process is finished, the host is replaced with an identical copy. In their base form, they are difficult enough to kill since they can easily absorb most attacks, but once they take over a host, they get access to all their skills and abilities. Obviously, this becomes a problem if they manage to infect a particularly strong or powerful host. The best possible outcome of this process is to end up accidentally creating an evil clone, but those were some very unusual circumstances. Most organisms don’t stand a chance when infected. Even power armor is no guarantee since the gooey form makes it easy for the parasite to slip into cracks.

The good news is at least x-parasites are not intelligent, at least not in their base form. They are driven by survival instincts, so they don’t rely on any sophisticated tactics. While the ability to imitate its host is frightening, there is also one upside- infected are easily identifiable. The x-parasite can look like the host, but it cannot replicate their emotions, instead relying more on muscle memory. While they can gain intelligence from each host’s memories, this favoring of brute force makes them at least easy to recognize when they do appear, which is more than can be said for some aliens. It takes some skill, but it can be stopped as long as one avoids direct physical contact. Their spread can also be reduced by the introduction of a predator organism, though this requires advanced genetic engineering.

7 Xenomorphs (Alien)

A Cruel Life Cycle That Can Only Cause Pain and Suffering

Xenomorphs are already terrifying enough in their adult form, where they are an apex predator. A single xenomorph can wipe out a group of humans with little difficulty. A group of them can be a nightmare even for a well-equipped garrison of marines. But as if that wasn’t enough, they also use parasitic reproduction. A xenomorph’s lifecycle begins with an egg that produces a facehugger, which attaches itself to anyone unfortunate enough to approach. They can also burn through glass so a protective suit won’t help. The facehugger then implants an embryo into the host’s torso, creating the chestburster which eventually tears its way out and grows into a xenomorph. If they manage to get a colony going, the process can be replicated on hundreds of victims simultaneously.

Fortunately, as gruesome as it is, it is at least possible, if difficult, to fight a xenomorph. Most guns will take one down without much issue, though their corrosive blood might cause problems. The facehugger relies on ambush, but it is possible for a prepared victim to resist, so long as it doesn’t latch onto their face. Even then, it is at least theoretically possible to surgically remove a chestburster before it can fully develop. There is also the fact that, while they can exploit existing technology, xenomorphs lack of means of getting off-planet on their own. This at least makes them containable, and a well-placed nuke might be enough to wipe out a colony, provided they don’t find a way to prevent it.

6 Seed Pods (Invasion of the Body Snatchers)

It Doesn’t Even Need to Touch You

The infamous plants that inspired the phrase “pod people” are yet another assimilation-based parasite, but they have a unique way of functioning. The pods create a body that takes on the features of a nearby potential host. When the host falls asleep, the duplicate body is able to download their memories and knowledge. When the process is complete, the original host dies and the copy assumes their identity. The new body is then used to further the spread of seed pods to more people to keep it going.

Most parasites need to make some sort of physical contact with the host to infect them. Seed pods have the advantage of not actually needing to make physical contact. Just being in proximity to one is enough to get infected. This gives them the advantage of discretion. They can infect large numbers of people without anyone even knowing of their presence, and without leaving a trace. While there is a simple way to identify who is infected, the sheer number of people it can consume unnoticed makes it hard to avoid. All it takes is for one seed pod to discreetly be placed near a victim, which becomes a lot harder to avoid as more hosts facilitate the production and spread of more pods. The one disadvantage they have is lacking a natural means of spreading. Instead, on relies on transportation used by the assimilated species.

5 Zerg (Starcraft)

A Swarming Menace That Consumes Everything in its Path

Parasitism can be a powerful evolutionary strategy, but few species have mastered it quite like the Zerg. In pursuit of evolutionary perfection, Zerg has developed a unique structure involving a hive mind that can assimilate other species into its control like an organic version of the Borg. Each species assimilated has its DNA absorbed into the collective, allowing the Zerg to evolve at an alarming rate. Every new gene sequence from every organism they assimilate allows the Zerg to experiment with different traits, constantly subjecting themselves to modifications and adapting to new challenges. This also makes reproduction really easy, so the Zerg can make use of overwhelming numbers to crush any resistance.

Fortunately, it is at least possible to fight back against them (if it weren’t, Starcraft would be over pretty quickly), but that is easier said than done. Humans and Protoss are the first ones to really put up any meaningful resistance, and even they can be assimilated into the Zerg collective. They have already left thousands if not millions of planets populated by nothing but Zerg-assimilated life-forms and continue to spread.

4 The Thing (The Thing)

One Cell is All It Needs

The thing attacks its hosts on a molecular level. Upon infection, it systematically begins overriding the host’s cells with its own, eventually replacing them with an identical copy. This makes it hard to eradicate the Thing, since doing so requires identification of who is and is not infected. A single instance, when identified or exposed, can be destroyed with explosives or fire, but even then it is still possible for a piece to break off and escape. As long as there is at least some cellular activity, it remains a threat. As if that were not enough of a problem, the Thing is also intelligent, with strategic prowess that would make a chess master squirm. It does not just attack people randomly, it chooses its targets with precision and exploits their growing paranoia of each other. It can also learn from its mistakes and sabotage potential countermeasures before they can be performed. The Thing even has enough technical knowledge to build vehicles out of improvised parts, providing a means of reaching new places to infect.

The thing is a danger on multiple levels. Aside from how it affects hosts and instills unhealthy amounts of paranoia, the Thing is theoretically capable of infecting and assimilating an entire planet, and it may already have done this on millions of other worlds. Once it gets going, it is nearly impossible to stop. Luckily, it can be quarantined, provided it is stranded in a remote location with no life forms and sub-zero temperatures, though the odds of surviving any attempt are basically zero.

3 The Flood (Halo)

A Parasite That Almost Destroyed the Galaxy

The Flood is a parasitic organism with a weird origin and a long history of terrorizing the galaxy. In fact, it played a big role in setting the stage for most of the conflict throughout the Halo games. Even it is initial feral stage, the Flood is capable of spreading quickly. Its use of spore-based reproduction makes it really difficult to eradicate with any reliability. Once they infect a host, the Flood can mutate their body into different forms to adapt to the current situation. What makes the flood especially dangerous is the fact that it can also learn from each host, so it naturally favors intelligent species. With each victim, the flood is able to absorb more knowledge and become more intelligent. Given enough time, it can reach the Gravemind stage where it starts acting like a co-ordinated army capable of advanced battle tactics. This intelligence also helps it find new ways to spread on an interstellar scale. Once that happens, there is almost no way to stop it.

The only reliable way to eradicate the flood is using the Halo Arrays, which also means wiping out all organic life on a galactic scale. This already had to happen once, when the Forerunners used the arrays after the flood had spread across the Milky Way galaxy and was on the verge of becoming an extragalactic menace. Although Master Chief can prevent them from being needed a second time, it came dangerously close and could theoretically happen again.

2 Brethren Moons (Dead Space)

Mass Infection on an Unprecedented Scale

These moon-sized aliens have a few tricks up their sleeve when it comes to parasitizing other races, something central to their reproduction. Their main strategy involves using markers as a kind of galactic trojan horse. These are thrown around all over the galaxy so they can easily be found by intelligent, spacefaring civilizations in need of resources. And who is going to say no to a free and infinite power source? Unfortunately, it’s a trap. Getting the markers inserted and spread out across a civilization is actually step one in destroying them from the inside out. Over time, the markers begin messing with the heads of organisms in the vicinity and mutilating corpses, turning them into killing machines, a plan that culminates in the destruction of their homeworld to create a new moon.

What makes the Brethren moons particularly dangerous is the scale of their parasitism. Most entries on this list work on an individual level. Even at their peak, most still work by individually targeting and infecting hosts. The Brethren moons can do it to an entire species at once. One only needs to be in proximity to the marker to be affected, and those who are not are surrounded by necromorphs who can rectify that. And the markers are designed to be easily replicable and a crucial power source, so trying to escape from one will just lead you to another. This makes it disturbingly easy to spread across an entire civilization, which is the point since its biomass is what ultimately creates a new moon.

1 The Hiss (Control)

A Multi-Dimensional Horror

The Hiss are an extradimensional parasite that managed to overwhelm the U.S. Government’s official paranormal wing. If not for the Oldest House’s lockdown, they would have quickly and easily taken over the world. Humans, even ones experienced with extradimensional threats, are pretty much defenseless without an HRA or personal protection from another entity, and even those have limits. Hiss infection can take several forms, but they all lead to the hosts being used as a tool to further its agenda. Sometimes this means performing incoherent chants that help spread it, and sometimes that means corrupting bodies into weapons. In a few very specific cases, the Hiss can get into a person’s mind and discreetly control them (sort of like Reaper indoctrination), which can be super destructive if they get someone powerful enough.

The Hiss are able to work on a level that most parasites can’t comprehend. Many of the previous entries on this list were capable of reaching dangerous levels but none have been able to independently spread across multiple realities, which also means more hosts than the Brethren Moons could imagine. It took massive foresight, scientific understanding of the paranormal, and intervention from an equally enigmatic being just to contain them. Adding to the danger is just how easily they overtake hosts. Most parasites on this list go through a transformation process of some sort. Hiss corruption often happens almost instantly. And they are non-corporeal so resisting them is even more difficult. This easily makes the Hiss the most dangerous alien parasite there is.