In the story I have been writing and asking questions for, the main conflict is parasites. What I have so far is that the parasite lives dormant inside of "pure-bred" aliens (which have softer brains), but once human genetics are introduced (alien-human hybrids), the parasite becomes active and begins burrowing into the brain of the host (which is more rigid, harder and more painful to get around). The body and immune system is completely unaware, as the parasite was developed in the brain during the fetal stages. Anyway, the parasite essentially creates its own alien brain tissue (as that's its regular "habitat") and creates a mess of alien and human brain tissue inside of the host's skull. The host begins having short spasms, memory loss, and feelings of being light headed depending on where the parasite does its work. The bigger problem is that the host also begins experiencing episodes of extreme violence. Characters in the story have noticed that hosts of the parasite are almost animal-like in how they act. This is because the parasite is in need of certain nutrients that only "pure-bred" humans can produce, so the parasite only targets humans. This nutrient allows the parasite to grow stronger and create offspring.

Leave a comment if you need more clarification or an example from my story. I am not interested in talking about the origins of the parasite. I only want to make it more plausible. I have done my research so I know the symptoms are alright, but I'm worried that if I write this into the story, there will be a lot of unexplained things/a lot of questions.

Additional information: If you could not already tell, this take places in a different solar system and it is in the future. I just want to explore these cool ideas, as we really don't know what's out there in space. But I don't want my ideas to sound stupid. I understand if I need to make major changes to the function of the parasite.

Edit: The aliens I am speaking of evolved in a similar way that humans evolved, and through science, they are able to reproduce with each other.

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    $\begingroup$ The biggest problem is how does it infect the fetus? If the parasite is in the brain, how does it get to the reproductive system? Is is constantly spamming out eggs into the blood stream? Is the blood infectious? Does it begin the re-wiring while still in the womb? Does it claw its way out? $\endgroup$
    – ShadoCat
    Mar 15, 2018 at 18:00
  • $\begingroup$ @ShadoCat those are all great points. I thought about the idea that the parasite would be doing just as you said: constantly shooting eggs into the blood stream. But I have been unable to think of how that would work, and so I just gave up on the idea. If you have any more you could say about it, that would be greatly appreciated. If not, thank you for your great questions, as Im sure they will aid me in coming up with more important details of the parasite. $\endgroup$
    – Alyssa S.
    Mar 15, 2018 at 18:42
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    $\begingroup$ There are two problems with shooting eggs into the bloodstream (in humans, at least): the blood/brain barrier and the placenta. I'm not a doctor or a biologist but if you can figure those out or handwave them because of "alien," you are good to go. $\endgroup$
    – ShadoCat
    Mar 15, 2018 at 21:10
  • $\begingroup$ As I mentioned in my answer, toxoplasma gondii is transmitted from the mother to the foetus. $\endgroup$
    – Hawker65
    Mar 16, 2018 at 9:56
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    $\begingroup$ Also remember that a parasite's job is generally not to kill the host. The longer the host lives, the longer the parasite lives. So any odd behaviors would result from the host not being wired correctly for the parasite. The violence and rage it produces would be accidental since it didn't evolve for humans and the way the human's brain is wired. $\endgroup$
    – ShadoCat
    Mar 22, 2018 at 16:32

5 Answers 5


You have fungi species called cordyceps that parasite insects and change their behaviour drastically. For the case of ants, once the fungus finds its way into an ant, the ant will tend to leave the colony and find a nice place for the fungus to grow (if possible above an ant colony to help the spread of the fungus).

There is also toxoplasma gondii, a parasite that makes the host attracted to felines. For example, mice infected by this parasite will not fear cats and will be somehow attracted to them until the cat basically kills and eats them. This is due to the fact the this parasite only multiplies once in a felid's body. By the way, this parasite is transmitted from the mother to the foetus.

Both of these parasites alter the host's behaviour in a somewhat drastic way. I'd suggest you invent a reason for the parasite to make the host violent, some way that would benefit the parasite.

For example, their aggressiveness could be explained by the fact that these parasites need human nutrients to grow and spread. Therefore, eating (and so attacking and killing) humans would be motivated by a goal (spreading and growing). You could make them attack humanoids in general if the parasite is not "smart" enough to distinguish humans from humanoid aliens on first sight.

They could also be weak to a certain thing (like fungi do not really like sunlight and prefer to be in the shade). Exposing the parasite to this weakness could trigger an "emergency reaction" in the host who will try all it can to avoid this weakness (ex: find shadows if sunlight is a weakness). In this "emergency state", the host could be very aggressive because it is in pain (because the parasite is exposed to its weakness), and it would do whatever it takes to get away from the weakness.

  • $\begingroup$ Toxoplasma gondii also infects humans (through cats) and there were studies to see whether they also modify human behaviour, even though we aren't its original target. I tried to find links but half of the posts say yes, they make humans more risk-taking and other things and the other half of online publications refute this. Whichever is the case, this could be true in this alien parasite: they do infect humans and hybrids but neither of them is its original target, so the behavioural modifications they produce could be either aimed at another species or be an accident of biochemistry. $\endgroup$ Jun 27, 2018 at 10:46
  • $\begingroup$ Exactly. There have been many conjectures stating that Toxoplasma Gondii is the origin of the huge popularity of cats on the internet. $\endgroup$
    – Hawker65
    Jun 27, 2018 at 10:54
  • $\begingroup$ Perhaps the lack of whatever the parasite needs that is not available in a hybrid, or at least not available in large enough quantities, could mean that the parasite excretes a slightly toxic chemical which causes stress and anxiety in hybrids leading to the violence $\endgroup$ Jun 27, 2018 at 13:41

Here's my take:

Your parasites aren't actually dormant in the pure-bred aliens. Rather they have formed a parasitic symbiosis with the aliens. If the aliens aren't affected or it's part of their brains as they grow, it's a part of them, something beign that's in every alien. Their squidgy brains just adjust, and might even use the parasite. The parasite in turn helps the host survive, maybe by giving them stuff such as extra brain power, a faster reflex system, structure in their brains that they would not ordinarily have without them and so on. If you killed the parasites in the alien's brain, they might die. And if they suffer brain damage, the parasites might even work to reconnect things, the better that their host will survive.

It gets passed through to the fetus from the mother. What this might mean is that hybrids with a human mom won't have the problem or hybrids grown completely in the lab don't have the problem. It could be that several generations were totally lab grown and they eventually figured out how to put it in the non-human moms, creating this unforeseen problem.

Regular humans visiting the planet aren't affected because their core temperature is too high or low or their body chemistry isn't compatible with the parasites.

But with the hybrids, there's enough alien DNA that they make decent hosts, but not great ones. And the environment isn't exactly hospitable. So while the parasite is present in the hybrids, some condition (like core temperature or body chemistry) keeps them turned off--for a while.

You want normal people to go suddenly bad, or live their whole lives until they one day start going mad--that's how to do it.

So in the hybrids it's actually dormant to start with. NOT in the aliens.

Genes can be turned off and on, mostly once they are on they stay on, but it's a messy process. It's possible that certain growth genes in the parasite are turned off to begin with, but some environmental (including food or anything else) or a hormonal factor turns them on, rendering them active.

When the parasite gets turned on in the human hybrid brains, the more rigid structure, which didn't evolve with this symbiosis will cause the problems you outlined.

The aggression is pretty specific though. In humans it would be sort of random how they would react--so you are going to need an explaination for all of them behaving in exactly the same way.

What I've come up with is this--the parasite is trying to build where it's easy. The brain of a hybrid might have a squidgy bit that gets activated by the parasite because the parasite believes that their host is sick and is trying to protect the host by turning on defenses. The parasite might be going to a specific point in the brain that will make it easier for them to spread over the whole thing. Brain structure can be pretty darn different...

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    $\begingroup$ excellent. The beneficial parasite is attempting to correct the human component of the hybrid. To make the story more interesting, you could vary the result. Some might have extreme violence, but some might have other extreme emotions. Or new emotions. $\endgroup$
    – Willk
    Mar 15, 2018 at 22:29
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    $\begingroup$ A good stage in the individual human for making the parasite active might be the puberty - suddenly everything changes and the weird hormone cocktails that are more extreme because of the alien DNA awaken the parasite. Depending on the exact specifics the hybrids may have a "human puberty" and a later, more random, "alien puberty" which introduces enough "alien" into the hybrid to awaken the parasite. You could then declare that "The physically stronger the later the alien puberty occurs" or something similar for varying timings. $\endgroup$
    – Secespitus
    Mar 16, 2018 at 10:08
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    $\begingroup$ @Secespitus Yes. This would be good. The problem I am coming up against is that hybrids can only be born through scientific intervention. The OP seems to imply that there's a pretty good population of them so it's going to have to be explained as to why they kept creating hybrids if this is the result. The later puberty might help with that because they started churning out multiple batches before they were aware of the problem--or maybe some kids have less than 1/2 of one set of DNA which makes a difference--like they tried to correct one developmental problem and ended up with this? $\endgroup$ Mar 16, 2018 at 20:14

Technically speaking, the thing that sets humans apart from other more primitive species is the neocortex. Also, humans can survive without parts of their neocortex and it can cause them to act more aggressively (see Phineas Gage). So maybe you'll accept that as evidence for my opinion that removing or damaging some of the neocortex would make humans more primal.


You're nearly describing the effects of many forms of brain damage, including Alzheimer's. Depending on the part of the brain where the disease (or in your case, the parasite) is developing, different faculties are lost - memory, language, self-control.

So there's not really a need for the parasite to "deliberately" affect any part of the brain to make particular changes - any sufficient loss of brain matter, whether by cell death, being eaten, or your scenario of being replaced by presumably incompatible matter, will cause similar effects.

Following the pattern, the violent phase would likely be followed by an apathetic phase as more faculties are lost.


So this parasite will zombify its victims and instill in them the craving for a specific nutrient?

Now, being it a parasite, we don't expect it to be smart, just doing its job: feed and reproduce.

The best food it can aspire to, living in our brain, is sugar. A lot of sugar, the more the merrier! The body methabolism sends the sugar first thing to the brain, so at first the victim would display headaches and a craving for sweeties. Before you know it, the victim is acting like an heroin addict, he's getting violent cravings for sweeties, he's going progressively beyond reason. Luckily, the parasite, who should give him the headaches of his life, is also interfering with his glandular system. He's getting continous adrenaline shots and his dopamine levels are off the charts. Luckily, the parasite is absorbing all the overdoses of sugar and for now the victim does not end up with a massive diabetes.

Towards the final stages of infection, the victim will start vomiting. Vomit contains spores that, once the liquid has evaporated are released and ready to infect other victims. The parasite dies, having disseminated its 'eggs', the brain collapses, the abused body soon follows.

What a sweet way to go!


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