Most science fiction stories and movies (from Star Trek to Close Encounters of the 3rd kind, ET, and many many others) assume one day we will eventually be able to contact physically the alien entities.
I mean: we can touch them, they can touch us, and we can interact freely inside the same physical space.
However (and even if they don´t talk about that in those stories), to arrive to that scenario, after the first encounter with a new alien civilization a "quarantine period" must have been established to confirm they have no pathogen microorganisms that may affect us (or vice versa). So my question about that matter is:
How long should be considered "enough" for a quarantine period before letting the aliens physically enter in our ecosystem?
Consider here on Earth we have diseases that have incubation periods of hours (Cholera) or even 10 years (the Kuru disease for example).

Please note I am not asking for the specific method of finding if they are dangerous or not. Perhaps the tests will be performed exposing bacteria to samples of their tissues, or exposing lab rats perhaps. I would like to know (independently of the type of tests) how much time should be invested in the "waiting to see if something happens".

  • 1
    $\begingroup$ Related - worldbuilding.stackexchange.com/q/50706/6781 $\endgroup$
    – Rob Watts
    Jun 30, 2018 at 3:39
  • 2
    $\begingroup$ Quarantining the aliens won't do much good, our biggest worry is the microorganisms they carry all the time, you would be quarantining the humans who come in contact with them at first and doing some tests but after that quarantine is pointless they will always carry the organisms in question. $\endgroup$
    – John
    Jun 30, 2018 at 15:11
  • 1
    $\begingroup$ What if they are the "virus"? Based on the evidence of what we'd done to our planet, humans could qualify as a quarantinable species on other planets. $\endgroup$
    – Bohemian
    Jun 30, 2018 at 21:19
  • 1
    $\begingroup$ It's entirely dependent on what technology is available. In Star Trek, it seems to take a mater of seconds (if that) for the computer to scan and detect foreign contaminant, especially if you use a transporter. In CEotTK and ET, there isn't enough information, other than that we know we don't have the tech to do what Star Trek does. IMO, this is too broad; pick a universe. Ours? Ours doesn't have FTL.... $\endgroup$
    – Mazura
    Jun 30, 2018 at 22:23
  • $\begingroup$ Considering contamination concerns regarding space probes sent by us in our solar system, one may want to even quarantine the vessel containing aliens in low earth orbit $\endgroup$
    – user35577
    Jul 1, 2018 at 21:58

7 Answers 7


Frankly, unless we have an already perfect knowledge of trans-planet viruses, diseases, and the like, the Safe Answer is simple: Indefinite.

But humanity is impatient, so more likely it will be in stages of risk, whereas for one example, more and more people are allowed to interact with them every 10 Earth1 years. So if it turns out a certain problem takes 30 years to reveal itself, then by that time even under this method, relations can still be salvaged and humanity not wiped out.

Note: The chosen humans are kept in quarantine away from the others humans who have not yet been exposed to these new lifeforms, to prevent possible spread of unknown problems.

So at very minimum 10 years, a reasonable time would be 100, and the safest would be never.


It depends on how similar they are to us

The ideal situation is actually that their biochemistry is a little bit different but not completely incompatible. Let's look at the two extreme cases to see why that's the case:

Case 1: Their biochemistry is identical. An example of this would be that a thousand years ago an advanced alien race took samples of humans and spread them throughout the galaxy. Then when we encounter those now-extra-terrestrial humans, we can assume that any diseases they may be carrying, we would be susceptible to. In order to be safe, the quarantine period would have to be as long as the longest-known disease incubation period.

Case 2: Their biochemistry is incompatible. If the aliens required cyanide in the air they breathe, the quarantine period is forever - you will never be able to freely interact with them without dying, as they would be giving off cyanide even through their skin to make any physical contact a bad idea.

The sweet spot is where we can survive with the same atmosphere, temperature, etc. without being immediately lethal to each other, but our biochemistries are still as different as possible. My reasoning for this comes from a previous answer I gave. In short, there are lots of bacteria and viruses on Earth, but only a small percentage of them are capable of making us sick. They are able to make us sick because they have evolved to be able to get around our bodies' defenses. With different biochemistries, it will be very hard for their bacteria to adapt to us, and for our bacteria to adapt to them.

In my opinion, the initial quarantine isn't the big problem. The big problem is what happens 100 or 1000 years down the road, when we have been living together with the aliens for so long that their bacteria have had a chance to adapt to us. That's when you need to be worried about a plague sweeping through humanity.

  • $\begingroup$ At the same time, our bodies have evolved to counter the bacteria of our planet. Alien pathogens may immediately infect us and make us worse than any bacteria on earth, because we are not adapted for it . $\endgroup$
    – minseong
    Jun 30, 2018 at 21:58
  • 1
    $\begingroup$ Wouldn’t we also want to be concerned about prions? There’s no guarantee the tiny life of a new world will hurt us but prions aren’t exactly alive and can stand to do as much if not more damage. $\endgroup$
    – Pleiades
    Jul 1, 2018 at 6:57
  • $\begingroup$ @Pleiades prions should be easier to contain - isn't it true that because they're not alive, they often don't have mechanisms to be contagious? I feel like the way to quarantine against prions would be fairly different than for bacteria - it's probably worth asking that as a question. $\endgroup$
    – Rob Watts
    Jul 1, 2018 at 17:22

OK, let's look at two of my favorite shows.

From the moment the invaders arrived, breathed our air, ate, and drank, they were doomed. They were undone, destroyed, after all of man's weapons and devices had failed, by the tiniest creatures that God in His wisdom had put upon this Earth. By the toll of a billion deaths Man had earned his immunity, his right to survive among this planet's infinite organisms. And that right is ours against all challengers, for neither do men live, or die, in vain. (Source, sorry for the lousy audio)

  • And from Warehouse 13, a paraphrase from Artie (I honestly regret not having access to a youtube of this quote, I about died laughing when I first saw it):

Would Thomas Edison have just picked it up? NO! He would have stuck it in a box! He would have wrapped it up and put it on a shelf until he was absolutely sure it wouldn't hurt him!

Therefore, there are only two plausible answers.

  1. Our medical technology is sufficiently Clarkean in nature that we don't need a quarantine.

  2. We never, ever, allow contact with an alien world, or ever allow anyone who'se visited an alien world to return to Earth. Never, ever, ever. And we put any aliens trying to visit our world in a box. Yup, and store it on the moon. Under moon rocks. And then we throw nukes at the moon rocks and aplogize to the alien diplomat via long distance communication later. Much later.

The one thing about H.G. Wells' War of the Worlds that no one ever thinks about is that all the humans would have died, too. The Martians would have brought their "tiniest organisms" with them.

  • 2
    $\begingroup$ Your quotation from The War of the Worlds is adapted from HG Wells' novel (upon which the movie was, loosely, based). Would the humans have died too? No. Wells thought of that, he explicitly said there were no bacteria or micro-organisms on the planet Mars. This was another part of reason why the Martians perished. So they couldn't bring any to Earth. $\endgroup$
    – a4android
    Jun 30, 2018 at 7:32
  • $\begingroup$ Unfortunately, it's not very plausible, because it works the other way around, too: All the micro organisms never had a chance to adapt to martian biochemistry, so chances that they would be able to infect them would probably be very slim. It's not like bacteria start out being able to infect everything and then everyone else needs to build up a resistence. Bacteria evolve to survive in certain ecosystems. On earth, a martian body is not among those ecosystems, so for bacteria, etc. a martian body would very likely be a deadly or simply uninhabitable environment. $\endgroup$ Jun 30, 2018 at 9:01
  • $\begingroup$ @FlorianSchaetz, you are having absolutely no fun with this whatsoever. Pity. $\endgroup$
    – JBH
    Jun 30, 2018 at 13:59
  • $\begingroup$ I think the idea is martian biochemistry was similar enough to an existing earth ecosystem that they were vulnerable to earth organisms that evolved under much higher competition. imagine if martian lung surface condition just happen to perfectly match tree bark surface biochemistry but they evolved in such a dry environment they never evolved a defense against mold, on earth the moisture in the air turns their lungs into a perfect breeding ground. $\endgroup$
    – John
    Jun 30, 2018 at 15:19

Answer to this question needs to take into account the context: while the visiting aliens might be aware of the sanitary risk (unless they are still stuck with the "pray and you will avoid sickness" sanitary method used by the westerners while exploring the Americas), it's not feasible to keep them put for ages waiting to be sure that they carry no diseases.

Also a similar concern would apply to them for themselves, with the addition of no world to back them up. So I imagine that the contact would take place in an environment set up to limit cross contamination as much as possible (no hugging and kissing, for example).

Based on this I would establish two separate classes: those who can come in touch with the visitors, and those who won't be allowed any contact.

The first set would then follow a progressively less strict isolation regime:

  • first 40 days after the contact of complete isolation in a strictly separated environment, to rule out quick onset diseases.
  • following 5 to 10 years of restricted contact with a limited set of non exposed people and frequent monitoring, to rule out slower onset diseases.

Meanwhile they could be monitored for "oddities" in their micro biome and health parameters, so that they could be released for public contact.

To be kept in mind is that 100% security can never be achieved, and a certain degree of risk has to be taken.


It all depends on too many factors, so let me offer another view, not mentioned yet:

  • There is another inteligent, skilled and ethical rase Threasan living on Thrae just couple of LY from Earth.
  • They live long, so they can wait for light-slow communication. The problem is, they use some other unknown to us kind, not radio, so they did not hear us, we did not hear them, even when our respective signals reach the other party long time ago, it was not noticed.
  • They have vaguelly similar biology as we have, so Earth was considered "habitable, but probably unhabitated" planet by them (and vice versa)
  • FTL travel is possible, but it is way expensive to enter "hyperspace" and moreover it is more like firing gun - nearly all energy is spend on the start and the fuel undergo some kind of chain reaction affecting all fuel in chain-range like thousand miles around - so it is one way extra expensive ticket now, but they can get a lot of baggage with them on the fly for not so big additional cost.
  • There is no exact evidence, that Earth is really habitable and usable for them
  • Sending lot of automated sonds would be too expensive (in planetary range too expensive) and their Artifical Inteligence is not as good as their natural inteligence (especially in unexpected conditions)
  • But they can send, say, about 10 Threasan (of both sex) and like year supply of food, air and such. And there is like thousands volunteers to one-way ticket (on the Earth would be for sure), willing to go even if they would die, but they would be HEROES and STARS, not mentioning all kind of possible scientific gain, they can collect there.
  • There is even chance, that the Earth would be habitable, in which case they can survive lot of years and wait for next colonist and establish "new Threa" here (and live to their natural death seeing their grandchilds colonise new word)
  • Also there is chance, that Earth would have a lot of unobtainium, so Thrae would send extra big and extra expensive second ship which could bring complete fuel factory and separated materials to compose fuel for way back and establish "both-way trade route"
  • and third, it is even possible, that the special fueld can be found on Earth in easily collectable (and processible) form, so they could return on their own eventually.

So one day with sound BLIP the Threasan ship pops out somewhere near Earth orbit (or not too far), with 10 aliens on board. They realise, that there is a lot of satelites, cities visible from orbit and such. They even found, that we try to comunicate, and while our radio technologies differs, we can use light reflections, pictures on ground (Nazca), or maybe lasers (not to cut them, but to direct light only on their ship), communication is somehow established. And now there is a problem what to do:

  • They can just orbit Earth for year, comunicate and heroically die, maybe even ask us to help direct their ship (and our support ships too) into the Sun, to evade possible contamination. Nice, but they will die near big source of fuel (say salt water, 1 cubic mile needed) in their reach (quarantine==infinity).
  • They can land somewhere anyway, hope to be able somehow work out secure contact and do not destroy any race. (And now it is on us, if we would cooperate, or if we just nuke them as invaders - and their next ship too)
  • Thay can colonize moon instead, we would supply them with some food, air, etc to sustain them there for life long, even helping them establish colony there. Our scientist (well morsels first) would travel (one-way ticket for start) there and make another colony near them, with third colony in between.

For start the third colony would be one-way-in for both parties, they would send their food and their morsels the, we would send ours and if both morsels survive for a month both parties coud send more food and one or two volunteers. If even they survive and be seen as healthy by both ours and theirs medical technology (and we can microscope all kinds of microorganisms their for both parties knowledge), then more volunteers can go in and mix both technologies and materials and food and such, in relatively safe way.

After couple of months of human/Threasans living in third colony without any (unsolvable) problems. As humans food (or other sources) shows to be eatable by Threasans, they can survive for life long, joining third colony and theirs (so far uncontamined) for more space and comfort at (small) risc of long term infection. Also more screw for our colony and more volunteers for co-living. Eventually our colony (still uncontamined) would split to part, which would return to Earth and part, which would join common living space.

With our resourses and their knowledge and preparations there would be small long term colony living on Moon.

After some time (and I would think about two or four years) when no ubeatable infection would detected by any medical technology it would probably be safe to create colony on Earth, in something like Area 52 (or Section 9) as well as create similat colony establishment on Noom orbiting Thrae. Their ship would refuel, (and as we know, that their food is basically eatable by humans too,) then without reserves for a year, but only for a days there can stuck a lot of humans and some Threasans for heroic return to their Noom colony to continue on discovering our mutual compatibility (now hosted by Thrae).

If after 20 years nobody in Area52 is hard alien/mutation-infected, it is probabely safe, as it means that we can live together long to have childs, living long to have childs and so on.

Also if there are some problems from start, we can work together on vaccination for that, as there are already both parties, one of them allready immune and having developed resistance, so there are many ways, how we can together fight the bacteria and make the affected party immunised too (or at least found how to suppress the spread of the bacteria).

If any deadly infection is detected in the way, that nuking Moon or even Area52 is still possible, but the longer, the more it would be probable, that nobody dies from infection, so even if there would be some, which would target some untested population (say those with some unusual genes combination), then there would be also big part population naturally imune (or able to be vaccinated against) such infection.

Well that is based on good will and cooperation of majority of Earth population, so it is sci-fi after all probably, but at least it could work there.

The main point is, that Thraesans, while having FTL are not having it so much to be able effectively overhelm us near Earth and humans not having FTL are not able overhelm them at their home.

On the other hand being in close contact with inteligent beings, just on Moon (or later on Noom) would lift a lot burdens of long time quarantine, making it only long time work at distant place, but still relatively free and surely not alone.

  • First, consider just how similar the alien biochemistry is to human biochemistry in your setting. Silicone biochemistry is being debated hotly -- would a silicone virus be able to thrive in a human?
  • Even on Earth, it is rather unusual to find a disease that can jump species.
  • Next, assume that there is an bacterium or virus that is endemic in the alien population which could infect humans. There would be a virgin soil epidemic that should be noticed pretty fast.
  • Last but not least, if biochemistries are compatible there is the chance that an infectious organism might be involved in horizontal gene transfer -- an alien plague and a benign human bacteria (or vice versa) might swap genetic information.

So I'd say a couple of months initial quarantine, followed by an increased risk forever.


How long should be considered "enough" for a quarantine period before letting the aliens physically enter in our ecosystem?

Why do you think we get to choose?

Naturally-evolved organisms capable of interstellar travel will be aware of the risk of infection. When they land and make contact, they'll already be confident that they are not at risk. They will either intend for us to be infected, so as to ease colonization, or they will have already experimented and felt confident that the risk of contagious disease was minimal.

Either way, our policies on the matter won't enter into it at all.

  • 1
    $\begingroup$ All those alien probes have been testing since 1947 so we should be good to go. $\endgroup$
    – KalleMP
    Jun 30, 2018 at 23:01

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .