Working on a story right now where parasite-controlled humans essentially become the apex predators in the world, and harvest uninfected humans to further spread the organism.

These kinds of mind-controlling parasites are not that uncommon in nature. But I want to take it one step further. I want true symbiosis between the parasite and its human hosts. It's easier for the parasite to do its work if the human body isn't always fighting back so it needs to provide some kind of benefit or incentive to the infected human.

What kind of benefits, in the form of physical enhancements/attributes most likely, could a parasite offer its human hosts?

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    $\begingroup$ Friendship. They'll never leave you alone like others in your life. $\endgroup$
    – airvine
    Oct 29, 2019 at 14:37
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Oct 30, 2019 at 0:28
  • $\begingroup$ Somewhat OT: Neal Asher has a thing for parasites. A major part of his polity series (and short stories) revolve around biologic and technologic parasitism. $\endgroup$
    – yankeekilo
    Oct 30, 2019 at 9:39
  • $\begingroup$ if it's a symbiote, it's not a parasite. $\endgroup$
    – ths
    Oct 30, 2019 at 11:47
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    $\begingroup$ Regarding: "It's easier for the parasite to do its work if the human body isn't always fighting back so it needs to provide some kind of benefit or incentive to the infected human." - note that this raises two almost unrelated issues: 1) "What can the parasite offer?" and 2) "How can a parasite/symbiot avoid being targeted by the human host defense mechanisms". Since humans don't have any conscious control over their immune system etc. the fact that an organism is perceived as beneficial has no bearing on how aggressively their body fights it. (continued...) $\endgroup$
    – G0BLiN
    Oct 30, 2019 at 11:58

13 Answers 13


First what you describe is a symbiote not a parasite.


What you describe already exists in humans.

What am I talking about Gut flora, or the slew of microorganisms that live in your gut.

It turns out our gut bacteria have a lot more influence over our behavior than we ever imagined. First discovered in mice gut bacteria modulate mood, cognition, even pain, but this has also been confirmed in humans.

without our gut bacteria humans are at a severe disadvantage, besides extracting a few essential nutrients they also help fight off infectious bacteria, even modulate the immune system directly. They may also play a role in development.

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    $\begingroup$ @Mindwin the OP makes no statement about the severity of mind control or even an example of the mind control. For comparison even things as simple as chlamydia have been shown to alter the host neurochemistry to make their spread more likely. $\endgroup$
    – John
    Oct 28, 2019 at 17:28
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    $\begingroup$ @Mindwin Actual "mind-controlling" parasites in nature don't actually take over the brain, they release chemicals that stimulate different than normal behaviors. Gut flora do the same thing. Since the OP references real parasites, this is actually exactly what the OP asked about... whether he realizes it or not is another matter. $\endgroup$
    – Nosajimiki
    Oct 28, 2019 at 17:28
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    $\begingroup$ Please note that most of these symbiotic organisms started as parasites or outright pathogens. However, an illness which kills the host quickly doesn't have such good chances to spread as the one which doesn't kill the host, so they evolve over time to be less deadly, and they might even become beneficial given enough time on evolutionary scales. If the paraiste gives health benefits to the host so that the host lives longer, it ultimately benefits the parasite as well. It's not in the interest of the parasite to damage the host, it's just a sign of an insufficiently advanced/adapted parasite. $\endgroup$
    – vsz
    Oct 29, 2019 at 10:54
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    $\begingroup$ The problem is a simplistic definition of symbiont/parasite leads to a false duality. They are two extremes of a spectrum, here it falls inbetween. E.g., if our vampire overlords protect us from alien invaders seeking our annihilation, then that's a benefit (and on balance, they're beneficial as the alternative is our demise) --- yet they're still parasites. By the same token, symbionts and commensals can have drawbacks too: e.g., the symbiont can be a vector for something harming the host, or it can use a resource (at a stretch: our gut bacteria consume heat, and water) --- still symbiont. $\endgroup$ Oct 29, 2019 at 13:45
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    $\begingroup$ @vsz I'd like to see your evidence for that. I think it's more likely that most healthy gut flora bacteria out as commensals: en.wikipedia.org/wiki/Commensalism $\endgroup$
    – user151841
    Oct 29, 2019 at 20:21

A couple of points that would make this parasite very attractive

  • Health. Simple. It comes in many forms obviously. Resistant to disease, faster healing, general health, better over psychical shape...etc.
  • Longer life span. Could be related to the first. That can also mean that both the parasite and the human have more time to learn and gain knowledge. Perhaps you can tie in that it takes them more time to mature than us so they have to give us more age.
  • More intense feelings and experiences. Imagine every experience magnified a couple of times. A bite of chocolate or a minute of music would be like an explosive feeling of absolute ecstasy. Though I'd recommend finding a way for people not to get bored or to start getting desensitized to feelings. Like how people overdose with time. So, be careful.
  • Better mental state and intelligence. Again pretty simple. If the parasite can improve the general intelligence that would mean a whole new world is open to the host. Also focus. The ability to actually buckle down and achieve something would propel the host in the world.
  • Removal of negative feelings including mental conditions. I mean the big tough disorders and absolute insanity or whatever mental problems that are really bad. Stuff like anger or sadness or even having a bit an extreme of a personality should not be effected. Just the hardcore stuff.

This is what I consider freeing the current world population of some of our biggest flaws while still maintaining who we are as humans. You can unify humanity under a hive mind, you can remove the sex drive from us, you can rewire us to despise all violence...etc. But those change are too dramatic to what makes us human. Though the ones above would produce a intense group of humans no doubt.

physical stuff includes but not limited to:

  • Stronger more adaptable overall bodies would be nice.
  • Better overall food handling system, I'm not sure what to call it, In the form of Not needing a lot of food. So more efficiency in handling the food. Then even if we eat, satisfying our great love of food, we won't get fat. Lastly less bathroom time.
  • Perhaps a faster maturity rate. Honestly having to wait something like 14-18 years to become an adult sucks. Historically we played around with the number but if we can just speed up the process of childhood and teenage year to something like 5-10 years for us to be in our 20's that would be great.
  • Redundant organs. Stronger bones. More resistant skin...etc. We don't really need this stuff for our current lifestyle. But if it's available then I'd prefer 2 hearts just for the heck of it.
  • Not aging. If death is still a thing then no problem. But at least in our 100th year we can be healthy as a 40 year old.
  • More beautiful humans. Does not need explaining.

Honestly the limit here is the imagination.

Now I think that the mental and moral aspects are more important and vital than having the ability to cross a desert. We evolved to be pretty good at what we do and we handle crossing the desert our way. But the biggest obstacles, especially now, are our heads and thoughts and ideologies.

You should also keep in mind that with every trait or added thing there is a trade off or at least a side effect. Longer life span, better health, and more sex would to more and more and more people. So perhaps the parasites would not want that or maybe they do. It's just a thing to keep in mind that there are consequences.

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    $\begingroup$ Since the op didn't specify how long the symbiote's lifespan is, I would suggest adding shared memories and feelings of previous hosts, leading to a more empathetic and knowledgeable host. You know, like Deep Space 9. $\endgroup$ Oct 28, 2019 at 14:00
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    $\begingroup$ "Not needing a lot of food" - for many people, I could imagine the reverse would be desirable - the ability to eat as much as they wanted without it affecting their weight. $\endgroup$
    – Skyler
    Oct 28, 2019 at 14:11
  • $\begingroup$ @Skyler Which route, then, could play an impact on how the world is structured and vice versa. A gluttonous people would desire or already inhabit the right world for it, and weak desire for food/efficient breakdown of food into nutrients would be beneficial in a more barren landscape. $\endgroup$
    – DVNO
    Oct 28, 2019 at 14:19
  • $\begingroup$ From my personal experience, "More intense feelings and experiences" can be pretty bad, depending on what they are.. And what seems constradictory, I would not really recommend lowering too much the bad ones, as they are probably useful, take the pain when you burn yourself for instance, remove the pain and you could burn yourself very badly before you notice it! $\endgroup$
    – Kaddath
    Oct 28, 2019 at 14:41
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    $\begingroup$ Most of what this answer describes sounds like the Goa'uld from Stargate. $\endgroup$
    – kojiro
    Oct 30, 2019 at 11:02

The host would feel good.

People become addicted to things that make them feel good - nicotine, caffiene, exercise (some weird people), chocolate. In your fiction, the parasite helps its host feel good. Hosts are confident, at ease, generally pleased with the situation. They are not incapacitated dreamy lotus-eaters, but persons with the parasite reset their sense of well-being higher.

There does not need to be a long term evolutionary advantage, but there might be - often an individual is his own worst enemy, thwarting his own success with worry, ruminations on old grievances, and self-sabotage. That could still happen with a parasite, but to a much lesser degree.

Humans suffer mental and physically, and they do not like it. If the parasite made people feel better in exchange for some calories and a ride, humans would not need to be harvested. They would line up to get a parasite.

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    $\begingroup$ Also, current <strike>victims</strike>carriers would just love to bring happiness to the rest of humanity. $\endgroup$ Oct 28, 2019 at 11:45
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    $\begingroup$ The parasite could do this by releasing psychedelics either in some predictable manner or that you can trigger it. Granted, there may be occassional bad trips, but psychedelics in smaller doses really seem to release emotional baggage. $\endgroup$
    – Juraj
    Oct 28, 2019 at 14:58
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    $\begingroup$ @Juraj - I was thinking more parasite Prozac, but for a fiction story your version is much better. It lends itself to a fiction because you could describe these psychedelic trips from the perspective of the host. $\endgroup$
    – Willk
    Oct 28, 2019 at 15:57
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    $\begingroup$ @StigHemmer Human culture has quite a few parasites like that too :P Though I suspect the OP only cares about biological parasites, rather than cultural parasites. $\endgroup$
    – Luaan
    Oct 29, 2019 at 8:14
  • $\begingroup$ Forget psychedelics. Pure dopamine is much more reliable and easier to manufacture (unless you go for the ethanol metabolic pathway, which is slightly easier to synthesize but tanks your host's long-term survival abilities) $\endgroup$ Oct 29, 2019 at 15:17

Easier, safer, and faster births

Operation Reboot Reproduction

One of the unfortunate things about how humans evolved is that our reproduction systems kind of suck. Infant mortality is a real problem (in some countries like Sierra Leone it can be almost as high as 10%) and even in first world countries, it’s unpleasant and painful.

Similarly unfortunate are the biological side effects of our reproductive cycle: menstrual products alone account for billions of dollars in expenses every year, and birth control methods, while extensive and varied, are often unpleasant and can have side effects. So let’s fix this.

...via artificial wombs

Extend the role of the symbiote from being merely inside its hosts to having an outside form as well. Fungal structures, ranging in various sizes, that serve as incubators for human children. Best of all, to start it up, the two lucky parents just have to get close: the sporogenesis process wraps their genetic material, and transmits to the incubator in little puffs of white smoke.

Meanwhile, the human reproductive organs are sterilized and altered in a way that removes the inconvenient side effects of having it, while maintaining the aspects that humans most enjoy.

Your symbiotic species can outreproduce its foes without ever needing to put any of its member or out of commission for that purpose.


Funny nobody remembered that: in the Farscape universe, parasites in the base of the brain made everyone polyglots, effectively tearing down the in-universe language barrier. Those parasites (or symbiotes?) allow (most) everyone in the galaxy to understand each other. As soon as you are infected, you can understand people talking in a completely unrelated language.

Every other answer here gave possible, "antropocentric" symbiotic relationships. Maybe a more fantasy edge would be beneficial to your story.

Edited to include @Luaan's excellent comment!

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    $\begingroup$ Could you elaborate on this answer? This answer does not seem self-contained. For example, I have no idea what the Farscape universe even is. $\endgroup$
    – overlord
    Oct 28, 2019 at 17:53
  • $\begingroup$ @overlord It doesn't matter, the answer explains the relevant point - there's symbiotes that allow (most) everyone in the galaxy to understand each other. As soon as you are infected, you can understand people talking in a completely unrelated language. $\endgroup$
    – Luaan
    Oct 29, 2019 at 8:16
  • $\begingroup$ @overlord. I feel like you've missed out ;) I recommend you watch Farscape! It's currently on amazon prime video. $\endgroup$ Oct 29, 2019 at 8:47
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    $\begingroup$ Farscape also had teeth cleaning symbionts similar to remora. $\endgroup$ Oct 29, 2019 at 22:30
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    $\begingroup$ @Overlord, thank you for adding the Farscape Wikipedia link. I assume (wrongly) everybody in this SE would know about it. $\endgroup$
    – Henrique
    Nov 4, 2019 at 17:40

Your premise was "It's easier for the parasite to do its work if the human body isn't always fighting back so it needs to provide some kind of benefit or incentive to the infected human."

The human body is unconcious and works on an evolutionary time frame. The human mind is concious and works on what humans would consider real time. You can provide many benefits that would appeal to the human conciousness, but the body is going to continue to reject any intrusion that it recognizes as foriegn until enough genetic mutation occurs to provide to create a mutually beneficial environment.

The rate and complexity of genetic mutation leads us to the truly interesting question. How did your proposed parasite ever become adapted to humans to begin with. If your reader is to believe that the relationship occurred naturally then you must begin with a terrestial, existing, human parasite. If the relationship occurred unnaturally, e.g. was directed by intelligence, what were the goals of the designers? The designers would have had a plan and an agenda from the beginning. What attributes best serve their agenda. Their obvious strategy will be to over-ride the human body's natural defenses and then to either persuade the human mind to accept symbiosis or to over-ride human conciousness and compell acceptance. Which route do your antagonists/protaganists choose?


Becoming an apex predator is the benefit.

Humans can live to be over a hundred years old so there's no particular rush for the parasite to concentrate 100% of the human's waking time to spreading itself.

If the parasite acts as a steroid without the adverse side-effects then men in particular would seek out this parasite assuming it allows them to control at least half of their life and "get all the girls".

Good luck figuring out how to stop these newfound apex predators from killing one-another in their spare time.

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    $\begingroup$ This doesn't seem like a complete answer. Do you have anything you can add to this? Otherwise I think this is a comment. $\endgroup$
    – Trevor
    Oct 28, 2019 at 17:54

Ironically, become the pinnacle of the human specimen.

It could make it possible for the host to consciously and quantifiably control their hypothalamus and general bodily glands. This would provide the following advantages:

Being able to regulate one's emotional state, feelings and mood.
In danger? Keep your cool on command. Need to figure something out? Focus on command. Sexual potency problems? Not with Mr. Parasite giving you an all-access pass to your own inner system. Feeling down? Not if you don't want to. Feeling calm but there's a fistfight on the horizon? Have some rage why don't you. Hungry? Have some dopamine to quench that hunger.

Being able to regulate your general health.
With full conscious control over your glands, you will be able to detect early if you are fighting some sort of virus or bacteria. Amping up focus in your glands could cycle the intruder out of your system quicker. Hypothermia, heat exhaustion? That's for non-parasite carriers to worry about! You can regulate your body temperature to whatever degree you so wish.

Super-human speed and power.
Having full control over your adrenal gland is pretty overpowered, no pun intended. Adrenaline reduces your sensitivity to pain. Varying the degree of release can result in a sort of permanent pain-blocker, depending on the situation. Don't feel a thing, whether it be arthritis or a bullet wound.

With these benefits it would be dumb not to invite the parasitic guests.

Possible downside:
Without all these natural defects, can you still be considered human?


One notion about aging is that bacteria have a much shorter life span than do humans. Some bacteria reproduce on a scale of a day or less. So over a human's life a bacteria may go through many thousands of generations.

The result is, the bacteria in any given human are evolving to adapt to the defenses of that human. Whatever your immune system can throw at them, the bacteria that survive are a little bit better at hiding from or resisting your antibodies. And over the decades they get better and better. Eventually they overwhelm your defenses. So there's a finite life you can possibly have because no matter what, you can't personally evolve. Your DNA is not changing very much over your life. Bacteria in you could evolve possibly 20,000 generations, so their DNA could be refined quite strongly towards getting past your defenses.

A symbiote could evolve. If you replaced your symbiote, say once every five years, then the new symbiote would have a new set of immune responses. The bacteria that had adapted for five years to the old one would now get destroyed by the new one. So that bacteria that had slipped by your immune system, and was learning your previous symbiote's tricks, would get destroyed by your new symbiote's immune system. The symbiotes that did not evolve to stamp out the bacteria would themselves sicken and die.

So in principle, symbiotes could provide a much longer life due to improved immune response. Not necessarily because the symbiote was particularly clever at immune response processes, but because it had a new bag of tricks more often than the humans it lives in.


Since you're working on an invasion story, I'd say that the main benefit of being infected could be immunity to a disease that the aliens spread on purpose.

Since it seems that you're wanting to harvest rather than eliminate the humans, I'd make the plague something incapacitating, but not completely fatal. Maybe something that makes people give off some sort of smell or other substance that makes them easier to locate...? Maybe laughing disease symptoms - make sufferers just break down and start laughing uncontrollably.

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    $\begingroup$ Well, it made me smile. Not a bad first post. When you have a moment please take the tour and read up in the help center about what's expected here. Welcome to the "site." (House of pain, pleasure-dome - take your pick these days.).. $\endgroup$ Oct 30, 2019 at 4:07

Super powers. Your parasite can give super strength or lightning fist or an number of powers. This lets your humans become apex predators by taking in the aliens.


Kill your enemies

(see how EU people invaded Americas)

Make you less vulnerable to something else

(not having a good example ready, but I am sure it is possible) edit: a parasite can make you unfit for military service or at least spare you from an unfavorable battle.

  • $\begingroup$ For making one less vulnerable to something else, this has been seen in real life - from the milder cowpox giving immunity to the deadly smallpox, to sickle cell anemia giving resistance to malaria. It might be immunity to a more deadly version of itself, or an immunity (or even just a resistance, if the whatever is dangerous enough) to something else going around. $\endgroup$
    – Megha
    Nov 3, 2019 at 23:59

No benefit. The parasite simply creates a permanent hostage situation such that the host cannot survive without the parasite. Plenty of real parasites turned symbiotic not by offering benefits, but by preventing harm.

From Nothing in Genetics Makes Sense Except in Light of Genomic Conflict:

In some species, like Wolbachia, the symbiosis can be obligate, such that the eukaryotic host (and all of its nuclear genes) dies when the symbiont is removed with antibiotics.


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