Imagine we're far in the future and mankind is part of a large web of planets/colonies including several other intelligent species. It's a space opera not much different from Star Wars. There is heavy exchange between the different worlds, moving billions of people and goods around. A large galactic civilization like this also means that new planets are settled/colonized every now and then.

Even without interplanetary travel, bringing species and diseases from elsewhere can be dangerous. Eventually, we became immune or learned to deal with the issue on Earth (mostly) but would it be possible to do the same with so much diversity? How can a world protect itself from the diseases of the others?

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    $\begingroup$ As this is something I've considered before, wouldn't the biology of different pathogens from different parts of the universe not be very effective against a given creature? Regardless, good question. I'm looking forward to the answers. $\endgroup$
    – Frostfyre
    Jun 30, 2015 at 4:35
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    $\begingroup$ Seeing the scale I have no doubt Murphy will strike...repeatedly. $\endgroup$
    – Bookeater
    Jun 30, 2015 at 6:30
  • $\begingroup$ That's assuming we are still being haunted by nasty bacteria and viruses meaning genetic modification of human as well as nanotechnology to inject pro/pre antibiotics must be still at its infancy then. Yes space is so vast chances of getting infected by alien plague really becomes a big concern, there must be mandatory advises and strict rules/protocols that all space colonists or tourists must adhere to failing which may result in prosecution. Fortunately assume wrap drive still debatable the infected either self aid or quarantine by vessel's A.I. en-route. Don't Panic n stay calm! $\endgroup$
    – user6760
    Jun 30, 2015 at 9:54
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    $\begingroup$ Robert Zubrin said it best: "Trees don't catch colds, and Humans don't get Dutch Elm disease". Second best line: You couldn't just replace Lions with Great White Sharks as the top predator in a Savannah ecosystem. Your main issue would be between worlds settled by the same species, where their co evolved diseases have had time to mutate just a bit. $\endgroup$
    – Thucydides
    Jul 1, 2015 at 0:03
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    $\begingroup$ Some answers imply limited or no problem since most troublesome diseases infect a small list of species, i.e., an alien disease won't bother human. IMO, that ignores the probably very small fraction of an uncountably high number of potential pathogens that will almost certainly make the alien/human jump at various unpredictable times. Also, once colonization is working, human pathogen mutations will complicate things. $\endgroup$ Jul 1, 2015 at 6:20

7 Answers 7


This is tricky since (presumably) life on other planets could follow other paths. But considering earth pathogens:


These are unlikely to spread to aliens, and vise versa. Virus reproduction is generally coupled fairly tightly with host cells - they use your own cell machinery to copy themselves. So foreign machinery is not likely to work.

However, because of the Birthday Paradox it's likely that if you had, say, 1,000 alien species, that a couple of them would have compatible viruses. But you're not going to see galaxy-wide viral infections.

This is good because viruses are harder to treat and defend against, so the fact that they shouldn't spread helps us.


Bacteria are less coupled to their host - they reproduce on their own. So it's likely that most bacteria would be able to spread to reasonably similar aliens. Thankfully, it's fairly easy to defend against with basic decontamination and cleanliness protocols.

Fungus and Molds

These are very dangerous because they pretty much just need moisture and some basic building blocks to grow. Like bacteria, for the most part they're not coupled tightly to host biology, which means they presumably can spread cross-species.

Defense is similar to bacteria - decontamination, keep things clean and maintained.

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    $\begingroup$ +1 But, not sure that the birthday paradox really applies. Even if all life is DNA based (fat chance), they would use different genetic codes and amino acids. It would probably be impossible for a virus adapted to one genetic code to ever hop to another. All this would only matter if the life arose multiple times in the universe and it wasn't some ancient species that contaminated the universe/galaxy with their microbes though. $\endgroup$ Jul 1, 2015 at 12:43

Honestly, I don't see this as being a significant problem, as already alluded to by other answers. There are a few reasons for this.

1) Diseases evolve to attack particular hosts, and as such can not easily spread to other hosts. Viruses are not going to mutate to effect non-human lifeforms; which won't even have DNA to 'take over'. Bacteria and parasites that are harmful to humans are ones that evolved to live within a mammalian body as well. They are not as tied to their host, but their still evolved to a certain enviroment and it's unlikely any alien species will have a body that is a good hosts. Keep in mind that odds are alien life forms won't even be carbon based, they won't have the right nutrients for parasites to 'eat' and will likely have internal body temperature, acidity, and materials so foreign that bacteria can't live within it. In short most diseases can not spread from human to non-human.

2) were already pretty good at containing disease. When is the last serious epidemic in a first world country that you recall? The closest was AIDS, and we almost immediately adapted to better use of condoms and other protection, testing those with it, and creating treatments to allow those with HIV to survive. Our science took a disease that could have been a population decimating disease (look at what it's done in some parts of Africa, where they did not have the same access to technology and information as first world countries) and allowed us to curtail it's spread, and treat it's effects; and this is a disease we didn't have means of fighting directly, we stopped it simply by identifying how it spread and preventing that agent.

I'm not saying that diseases don't stop happening, HIV is still around and hardly trivial, but it's also no where near pandemic levels. The further you put us into the future the more informed we will be about disease and how they spread, the more capable we will be at creating vacines or even anti-virals to combat these diseases. Put us far enough in the future that we have such covenient space travel and we will likely be able to combat diseases with direct treatments for most new disease. However, even if we can't we can control them and keep their spread limited quite easily simply by indentifying how they spread and stopping that spread.

The only disease I see being a threat would be one that lied dormant for an extensive length of time, to allow spreading to a huge percentage of population before we identified it and started to fight it. However, disease doesn't do this, but it's very nature it will always spread as fast as it can, it's not 'smart' enough to know to lie dormant to spread that long. Perhaps more accurately, any disease that spread so successfully while dormant would not become lethal all at once, dormant spreading is proving an evolutionary advantage over killing the host, why should it then adapt to being lethal? (and yes, I'm overly personifying disease and their 'decision' to adapt, but I stand by the the raw arguments as true, even if I'm skipping over some of the specifics of unguided nature of evolution to provide a simpler explanation).

In short, we have means of stopping spread of diseases now; will have even better means of directly identifying and detecting diseases later, and they will only be a threat to one species at any time.

furthermore, the enviroment you hypothesis would make controlling disease easier. Each world is still mostly cut off from the other. Yes you said that travel was common, but each world is an isolated unit with very controlled 'borders' for people to come and go. If one world is infected it can be isolated much easier, and even if it's disease spreads you can identify likely worlds to qurintee easier. You can enact policies to screen people coming from known danterous worlds and since all travelers will have to come via ship that enter spacedocks that are controlled you have a very effective way of screening to ensure no one enters a new world with disease; as opposed to now when someone can walk across a border from a city to another in any of a million paths and bring disease with them.

Furthermore if aliens are really intermingled with humans you get a quasi herd-immunity as well. Since the aliens can't catch the disease they are immune. If worlds are culturally diverse then you may have only 10% of your population being made up of any one species, meaning 90% of the population is immune to disease. Your get at least some limited benefit of herd immunity this way.

The biggest danger would be not from diseases that live in and attack the human (or alien) body, but from outside containment that are universally dangerous. Radioactive materials, invasive species and 'parasites' that don't live in the body, but around it and are harmful (imagine space fleas that evolve to live in material commonly used by space ships and can damage instruments or the like)

Still, even in these cases qurtine techniques would work well, and more advanced science means better detectors for such threats.

Thus the only possible pandemic level threat I could see are man(or alien) made threats. Genetically engineered diseases or a nano-plauge. These diseases could be designed to be lethal, most likely by having them spread for years before they become lethal, so they have time to ensure everyone catches them. Nano-anything has a theoretically horrible grey goo worst case scenario, be it intentional or accidental. However, in the end I think man would have to be involved in creating anything that could prose a serious threat to a species.

Of course diseases will still exist, and spread. Pandemic level disease won't occur, minor diseases will always be around, even lethal diseases will spread to some degree, just being contained to a mall number. Once a disease reaches a certain danger level humans will invest effort into stopping it, and will; but diseases below a certain threshold won't be worth the effort to contain. Even contained disease will still take lives as well, look at HIV in the US now, it's 'controlled' and unlikely to be a serious threat to out population as a whole, that doesn't mean it doesn't take a number of lives.

  • $\begingroup$ The infection may not be human/alien. A sulphur based alien may be perfectly safe when treating a human plague, yet develop a nasty case of Desulfonauticus submarinus when they go for a pleasant swim near Terra's cozy 400C deep sea vents. $\endgroup$
    – gmatht
    Jul 1, 2015 at 8:14
  • $\begingroup$ @gmatht I'm sorry, but I'm not sure I understand what your saying in your comment? I don't know what you mean by a infection being, or not being 'human/alien'. $\endgroup$
    – dsollen
    Jul 1, 2015 at 14:49
  • $\begingroup$ I mean that the alien could be infected by contact with a non-human Terran lifeform. $\endgroup$
    – gmatht
    Jul 3, 2015 at 3:37
  • $\begingroup$ @gmatht ahh, I understand now, but I'm afraid I would still have to argue against the comment. The exact same justification applies. It's unlikely an alien will have evolved to be anything like earth creatures, be they human, zebra, or firefly. It's quite unlikely any disease evolved to attack or parasite terrain species will be infectious to any alien species. $\endgroup$
    – dsollen
    Jul 3, 2015 at 4:50
  • $\begingroup$ Well, I understand that this has not specifically evolved to attack humans: healthed.govt.nz/resource/keep-your-head-above-water Clearly it wouldn't infect a creature that didn't have, say H20. But it sugars aren't very complicated so an alien with sugar water wouldn't be so surprising, which seems enough to keep a yeast happy. Still unlikely, but there are 300 billion stars in the Galaxy and the immune system would be unprepared. $\endgroup$
    – gmatht
    Jul 3, 2015 at 14:25

I'd like to introduce a few more vectors:

  1. Nano-materials; low chance of problems, highly likely to spread. Nano productive systems, once they evolve, enlarge the risk. Decontaminate.
  2. Prions; low risk but potentially bad consequences as it tends to move slow. Verify sources and spot check.
  3. Pests (a famous example is of course the Tribble). A valued domesticated resource can become a serious pest in another environment. Research before importing. Decontaminate. In case of trouble send for Capt. Kirk.
  4. Parasites. Heh. Alien. Send out automated decontamination robots first. Avoid space derelicts. Really avoid space Sargasso's.
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    $\begingroup$ When it comes to #4, there's really only one way to be sure. $\endgroup$ Jun 30, 2015 at 15:41

I would guess that many diseases would not pass between different species hosts. Many things that kill trees, have no affect on animals, and what kills a fish doesn't even notice a human and vice versa.

So many will not be transmitted between or communicable from one species to another. Some things that are such as some parasites might not care as much about it's host, while others are extremely specialized.

I would also think that technology would be improved greatly to help identify dangerous organism to help stop their spread. Ships air systems should have analysis units identifying any contaminates that are dangerous at a minimum to the species inhabiting the ship and any that are at the destination of the journey.

I would also think anyone who can afford it (and if your are traveling intergalactic you can't afford not too) would have personal units monitoring their bodies and environment as well as be in communication with the ship to be on the look out for organisms and chemical compounds that are dangerous to themselves.


As several others have said, humans are not likely to catch alien diseases. There are just too many ways to put together a working biochemistry and most of them would be incompatible.

However, humans alone could feed a pandemic if the conditions are right.

Scale doesn't really matter here. You have large populations brewing new diseases, and a small amount of travelers carrying these to new unsuspecting communities.

This is exact the same as the situation on Earth historically and today.

Medical technology will probably be much better, but I suspect that things will be as they are now: Only the rich can afford it.

So, pandemics will spread among the poor.

Travelers will generally be rich, meaning they will have access to vaccines and everything.

If travel gets cheaper to the point where you get migrant labour, watch out. Populations will truly meet for the first time in a long time.

An important question is whether information travels faster then people. If so, a bad pandemic in one place can be kept contained as warnings spreads ahead of the refugees. If the refugees get there first...

So, it all depends.

  • $\begingroup$ This answer is exactly right. Diseases spreading between species - unlikely to be a problem. Diseases spreading between planets colonized by previously long-isolated populations of the same species - huge problem. That is how the Black Death arose, and the reason why the South American continent was ravaged by European diseases in the sixteenth century. $\endgroup$ Jul 2, 2015 at 13:26

To avoid previously unknown deceases and new pathogens on a new alien world, civilization could use terraforming, to ensure only known organisms are present. Also this new world won't be open for free travel before a period of observations and trials on a limited population of settlers.

But even known viruses mutate and change, so after a couple of centuries each distant world will probably have a slightly different deceases and immune systems evolved. To ensure people won't carry something new with them every time they travel, quarantine and decontamination procedures can be applied for new arriving guests (probably there would be a list of similar worlds, with appropriate procedures for each group).

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    $\begingroup$ "Finally, we have the hyperdrive, let's explore the universe! But first, let's bomb the shit out of it. You can do your exploration when the surface is glass just fine, doctor Xenobiologist." $\endgroup$
    – Luaan
    Jun 30, 2015 at 8:11

They would protect themselves much the same way we do now in 1st world countries. The US has the Center for Disease Control (CDC) the Food and Drug Administration (FDA), Border Patrol and a couple others to manage epidemiological threats.

In an FTL capable society, they should have much better models for the spread of disease. Combined with far better sensors and analytical equipment, and the computational horsepower to model biochemical interactions, they should be much better off than we are. Likewise medical science should be better able to diagnose and treat illnesses.

The space equivalent of Border Patrol would have a huge role in keeping the various ecosystems separate as they will keep contraband biologics out. For an example of how and why, have a look at the scene in Fifth Element when the star cruiser is about to leave the city and undergoes decontamination.

Any product being shipped will need to be certified clean or be of a nature where no cleaning is needed, ie, no one cares about decontamination of 1 megaton of steel bars. When questions of contamination arise, institute quarantines. Ellis Island in NYC is an excellent example of this (minus the degrading name changes and family separations.)

Tldr, they would do it the same way we do now only with more science, better tools and most importantly, it's in space!


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