The background is that Earth is the victim of a massive comet strike, that wipes out a large portion of humanity (let's say 80-90%). Let's say that the comet was carrying an alien parasite that's a kind of super-extremophile, managing to survive both the cold void of space as well as the extreme temperatures from the comet impact.

After the impact, with the Earth in ruins, how does this parasite spread among humans? The goal for this parasite is to multiply using humans as hosts. What would be the most effective way for it to find the first host so it can start multiplying? Let's say that by utilizing the host, the organism can multiply and then pass on the offspring organism into other humans.

The issue is that now there are considerably less people and they are spread much further apart. Is it possible that maybe an expedition was sent to the impact site a few years after impact, once the Earth has stabilized a bit? Maybe the expedition is from a large settlement and they went to check to see if they could get raw materials, only to come in contact with the parasite? They would then bring it back to their settlement, where it now has an opportunity to spread.


  • $\begingroup$ Sounds like you've already answered your own question. Ever watched The Thing? $\endgroup$
    – Priska
    Commented Oct 9, 2019 at 18:24
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    $\begingroup$ @Priska Well he won't know if he's seen it if you don't say the title. /s $\endgroup$
    – Ranger
    Commented Oct 9, 2019 at 18:40
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    $\begingroup$ The spores of the parasite have waited for millions of years on the comet... They can wait for a few millennia to be eaten by a human. $\endgroup$
    – AlexP
    Commented Oct 9, 2019 at 19:15
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    $\begingroup$ Spores are probably the way to go. Also, extremophiles aren't likely to live in humans, because humans aren't generally very extreme. Resilience of spores is not the same as thriving in harsh environments. $\endgroup$ Commented Oct 9, 2019 at 19:37
  • $\begingroup$ How would the parasite be capable of living in humans? If it is an evolved organism, then evolving to be capable of infecting a creature not present in its original ecosystem would not provide any advantages, so it would probably be unable to infect humans. Our biochemistry would probably be totally different and our bodies would probably either provide no nutrition to it or be poisonous to it. $\endgroup$ Commented Oct 10, 2019 at 19:34

4 Answers 4


The first contact is always a matter of chance or luck, of course there's no story until it happens.

The downside in this specific case is that the level of destruction around the initial impact site is so high that it's going to keep humans well away for a significant period of time.

Of course there's no story if the parasite doesn't find a host so it'll have to get there somehow.

What are the odds, a million to one?

It's going to have to be a mobile parasite, to survive that sort of environment and then the impact we're talking about a microscopic lifeform, nothing macro could survive. That means it could potentially be windborne but it's unlikely, waterborne is a better option, that gives it an environment in which to reproduce while awaiting a host.

Let's reduce those odds a little

It's an extra terrestrial parasite, that means to infect humans it's going to be zoonotic anyway, so let's make it zoonotic.

Now it's just a matter of what we make it able to infect. A water impact for your asteroid would be most deadly to human life, the easiest way to get your parasite into humans is to get it into the food chain.


Probably the easiest path from the open ocean to humans is via salmon (and some trout). Fish that go to sea during their lives then come back into the rivers to spawn. That gives you fast distribution over large areas. Make the parasite transmitted through sperm as well as transmitted to the eggs means that you'll get rapid distribution through the fish population over a few years and hence easy transmission into humans.

The infection of humans is now a minor detail in the existence of this parasite, mostly only of interest to the humans themselves.

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    $\begingroup$ Frozen in ice on the comet, sent into a body of water upon impact sounds good $\endgroup$
    – Whitehot
    Commented Oct 10, 2019 at 15:03
  • $\begingroup$ It could be a collection of microorganisms that conglomerate into a Macro collective, like a slime mold. $\endgroup$
    – Brinstar77
    Commented Mar 11, 2022 at 15:52

Make the comet unusual and therefore interesting. Human beings, even apocalypse survivors, are inherently curious creatures.

Let's say that the comet was identified as earth-bound months before its arrival but was downplayed in the media as being too small and lightweight to survive atmospheric entry. Everyone expected it to burn up without impact, so when it slammed into the Earth with cataclysmic affect, those who survived are left to wonder what happened.

With that kind of a set up, a human exploratory team would head out for the crater long before the dust settles.

Take a look at The Ice Limit by Douglas Preston and Lincoln Child for a brilliant explanation for the comet's unusual attributes. If a comet such as the one in their story were to strike a weak spot in the Yellowstone caldera, a 20% survival rate could be very believable.


A few scenarios for you to choose from:

  1. Humans and parasite start at the same location

Dunno why this would be, but maybe the comet fragmented on arrival and the parasites lands in a populated region. Maybe just outside a city, or in a rural area. Contact is inevitable.

  1. Humans and parasite start far apart, humans go to parasite

As you said, maybe the humans are curious as to what the comet actually was, or go scavenge around the deserted impact site to find resources. Contact occurs when humans approach it too closely.

  1. Humans and parasite start far apart, parasite goes to humans

This could occur for different resons, depending on what your parasite is. If it's spores, most logically they get carried by the wind & rain to a human settlement and infect people like that. If it's alive as it lands, maybe it can smell humans from a long way away and hunts them down. Maybe it just wanders aimlessly and happens upon them. Another possibility is that it infests a different host, maybe a wolf or something similar, and wants a better and more intelligent host (or that can handle tools?) so it decides to track down a human settlement.

  1. Humans and parasite start far apart, parasite and humans converge towards one location

Sort of a combination of 2 & 3. Maybe after ressources get scarce, a settlement of humans needs to relocate towards a source of fresh water, just like the parasite, whether it's found a temporary host or not. Could be quite an interesting build up to the human / parasite contact if that's what you want to see.

If anyone has other scenarios that don't fall into these 4 categories, tell me and I'll add them.

  • $\begingroup$ This is a reasonable break down to several possible scenarios, however, OP asks for the most effective way for the parasite to infect humans - both how to hit "patient 0" and how to spread to multiple hosts later. This post doesn't really address that. Consider focusing on one of the four scenarios and expanding it - covering how it would work and why it is more effective than others. $\endgroup$
    – G0BLiN
    Commented Oct 10, 2019 at 15:25

It depends on several factors:

  • is the parasite intelligent (whatever intelligence might mean)?
  • how mobile is it?
  • what sensorium does it have?
  • what kind of effect can it have on the host?

Let's start with a reasonable intelligence (above that of a dog), plus a sane dose of instincts.

Intelligence (and instinct) might help it survive the impact. Contrary to what you say, no extremophile, whatever "extreme", can hope to survive a cometary impact. Temperatures are way beyond what a carbon based lifeform can withstand for any length of time, and in the center, they're going to stay that way for an appreciable time; plus pressure and shock is enough to homogeneize any biological material (just before flash-frying it).

But about a minute before impact, temperatures are survivable and the parasite might be thawing while the comet gets ablated in the upper stratosphere. Seizing the opportunity, our parasite can escape the comet - by letting itself be blown away by the air currents. Shaped like a ragged bedsheet, the parasite can fly away and escape the impact and the subsequent tephra, the firestorm and the smoke, and let itself be carried far and away.

First thing, it "knows" it has to find food and/or hosts. If it is reasonably mobile and has a useable hearing or sight (either for daytime or nighttime), it can home on movement or light or noise or body heat.

Then, if I were it, I would dedicate a long time to regrouping and assessing the situation. It has to adapt its biochemistry to that of Earth, at least to a point. It has to decide which host would be best.

During this time, civilization collapses in post-apocalyptic chaos.

One interesting opportunity for our parasite might be that of becoming a symbiont. Like Hal Clement's Hunter (and somewhat, W. M. Miller Jr's Dark Benediction), in exchange for a home - a human body - the creature might maintain that home. Depending on its capabilities, infection by the parasite might be actively sought after. Imagine if having a "companion" could allow better chances of surviving the post-apocalypse (a Hunter capabilities extended to helping its host recover from a steel spike being driven through the heart, or surviving without drinking by removing salt from seawater).


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