I won't claim to be a biologist, but I'm fairly certain that it's really hard for disease to survive in space because most bacteria and viruses need a host in order to survive for long periods of time.

So, let's say a group of very healthy astronauts were sent out into space. Their only contact is with one another and they reproduce. Their children wouldn't come in contact with many childhood diseases, and while they may inherit some antibodies from their parents, their immune systems would probably take a slight hit after not needing to fight off any of these diseases.

I'm assuming that some diseases that were on the ship at take off would mutate in order to survive, but the vast majority of human illnesses will never make it on board.

How long will it take for the immune systems of this group of people (and their descendants) to deteriorate so that they can't return to Earth without dire effects to their health? Would this even be an issue?

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    $\begingroup$ A major factor is how many astronauts, in a large generation ship they may very well come back with new ina interesting diseases. the high population densities of european cities is believed to be a major factor in why they had more devastating diseases. The other being livestock. $\endgroup$
    – John
    Dec 13, 2016 at 18:22
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    $\begingroup$ Based on the wording of the question, it sounded like something was immune to humans. $\endgroup$ Dec 13, 2016 at 19:01
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    $\begingroup$ One factor to consider is bacteria grown in space become more deadly, so people in space may not last as long as one might expect. $\endgroup$ Dec 13, 2016 at 21:01
  • $\begingroup$ Also, DNA can survive reentry and Tardigrades can survive in empty space. Perhaps other strains of bacteria or viruses can too. $\endgroup$ Dec 13, 2016 at 21:04
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    $\begingroup$ You may not be considering that every one of those astronauts is carrying a large infectious agent machine in their gut, one which has demonstrated rapid mutation and horizontal gene transfer... $\endgroup$ Dec 14, 2016 at 1:33

7 Answers 7


You can't ask "how long" in years, but in generations.

And the answer is One Generation.

We have two kinds of immunity... innate immunity and acquire immunity.

The first one (innate immunity) will react to a multiplicity of organisms. And it will react always the same way.

This is the first line of defense of your organism, because it's the simplest and the fastest. Your space travelers would also have this kind of immunity, so they wouldn't be completely immunosuppressed.

Trouble is... this isn't a "immunity" per se, because people don't get immune with this kind of response. Because when you are exposed to the same kind of microbe, you'll need to develop a immune response all over again. You can get sick with the same disease over and over and over.

Acquired immunity... That's the real immunity. Because your body learns how l fight that specific disease.

When exposed to a certain microbe, the acquired immunity will develop a response appropriate to that specific microbe. It will then memorize that response, so that when exposed to the same microbe it will elicit a fastest and more efficient response. You won't get that sickness ever again.

Now, as the name states, you can't inherit acquired immunity. You have to develop this responses by yourself, by being exposed to the pathogen (either the real deal, or an atenuated surrogate, aka vaccines).

Infancy is a special time period for this immune "training" to develop. If you don't, your immune system won't know how to adequate respond to stimuli. People raised in aseptic environments are prone to develop inappropriate immune responses to inocuous environmental stimuli, i.e., allergies and autoimmune diseases. (Your space travelers would suffer from this, BTW)

As soon that you have a generation reaching adulthood and that never had any contact with common earthly pathogens (even via vaccination), then they would have a lot of trouble fighting even common diseases

All this, assuming that there isn't any other variable immunosuppressing your space travelers. Perhaps chronic exposure to space radiation affected their blood marrow's ability to produce immune cells (either innate or acquired)

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    $\begingroup$ Yes, this is also why every baby (or nearly every baby...) in the United States (and many other countries), for example, gets over two dozen immunization shots over the course of their first year of life; those immunizations are not inherited from the parents. $\endgroup$
    – TylerH
    Dec 14, 2016 at 15:27

I'd suggest that they may lose at least partial immunity to a decent extent within a decade or two of leaving Earth.

According to Smith et al. (2014) (explained for us laypeople here), the number of disease outbreaks has been rapidly increasing over the last few decades:

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Specifically, there's been a large increase in bacteria- and virus- carried infections.

I'd argue that this could be quite a problem. Immunity to past strains of a disease can be created through vaccinations, and I see no reason to not bring enough on the ship for future generations. There's no sense in having a child somehow get chicken pox when they go to Earth just because someone felt it wasn't worth sending an extra syringe along.

The odds of this are extremely low, assuming proper procedures are followed to make sure that no diseases manage to hitch a lift on the ship. But with 3000 outbreaks happening over the span of five years - many likely from new varieties of illnesses - it seems highly likely that the descendants will not be prepared to deal with any of them once they leave space. The only way out would be to constantly supply the ship with vaccines from the latest strains of disease, or to ensure that the spacefarers undergo quarantine when they come back until they receive the correct vaccinations - not bad, considering the odds of them contracting, say, Ebola while in Earth orbit are slim to none.

So, without vaccinations in space or back on the ground at arrival, I'd say that within ten years or so, they'd be incapable of dealing with a variety of new strains - many minor, some significantly worse - when they go to Earth.

I'm aware that the question asks for "dire effects". Diseases which can have nearly guaranteed severely negative effects on most members of a population don't necessarily follow the same trends as shown above and can be difficult to anticipate in advance. If a disease that becomes the next smallpox strikes Earth within ten years, this answer becomes very different. But if we don't see such an epidemic within a few decades, then the answer is simply that there won't be any significant problems.

I don't think I'm qualified to speculate on when the next pandemic will hit, so I'm not going to try.

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    $\begingroup$ They might get sick upon landing, but would it be life threatening? Almost certainly not. If that were the case, most international travelers would keel over dead. Most of us instead plan to be sick the first couple days in a new place and then our bodies have compensated for the local micro fauna. I think this bears further research before being accepted as an answer. $\endgroup$
    – SRM
    Dec 13, 2016 at 15:00
  • $\begingroup$ @SRM There are very few illnesses on Earth that could produce that sort of effect in any population, and it's not easy to predict when a new one will arise. I'm not willing to speculate on when such a disease could strike Earth, so I went with a more conservative estimate of when their immune systems could be threatened, at the least. $\endgroup$
    – HDE 226868
    Dec 13, 2016 at 15:02
  • $\begingroup$ Like I said, I think we need to hold out for more data from someone else. $\endgroup$
    – SRM
    Dec 13, 2016 at 15:13
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    $\begingroup$ @TheNate But can it kill people over 4050? $\endgroup$ Dec 13, 2016 at 23:37
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    $\begingroup$ Heh... Yeah, that would be my phone. I don't know why it doubles text like that sometimes. Irritating, but at least it doesn't explode violently, so life is good. $\endgroup$
    – The Nate
    Dec 13, 2016 at 23:52

Yes. Remember, this is what killed most of the Native Americans in America was "discovered." There were many diseases such as the flu that they had never been exposed to in any form, and they killed a large percentage of the population. The ones that lived and developed an acquired immunity survived and reproduced.

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    $\begingroup$ Yes, that exact event was actually the inspiration for this question. I was actually wondering how long it would take for this sort of thing to happen. $\endgroup$ Dec 13, 2016 at 17:31
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    $\begingroup$ Somewhat true, but the fact that Europeans had lived with these diseases for centuries was also an important factor. Evolution had favored people who were not very vulnerable to these diseases, so individual exposure was not the only issue here. This type of genetic immunity would stay with your astronauts for many generations. $\endgroup$
    – user2727
    Dec 14, 2016 at 7:38

The real answer depends on the illness. I am not a doctor of any kind, so these are just based on my observations.

First level, are things that change every so often. Like the flu. You have to get a flu shot every year to stay vaccinated because what actually happens is the flu virus changes etc. These would happen almost instatly like it does today.

Second level, things that we need booster shots for. Tetanus, for example. With those shots are immunity wares off. TB, and Hepatitis are also good examples.

Third Level, vaccines that we all need to get once. MMR, Polio, etc. These would last 1 generation.

Final Level, hereditary immuneities. There are some. But not many. Mothers pass on a few antibodies, but they are more of the "general" kind and not the specific kind. Think of them as "antibiotics" and not "the anti-body that fights xyz infection". Usually by the time breast feeding is done, those antibiotics are wearing out in the infant. But the infant has produced some of their own. Without exposure to boost antibody production against a specific infection, then even the antibodies that get passed on would start to fade after a single generation.

So without exposure to there viruses and bacteria after just a few years your astro-dudes could be in some serous trouble (check NASA documents about long term space habitation for some examples). After a few decades, they could end up with some serious infections, and after a generation or two, they would not be able to interact with us "land lice" without some serious medical intervention.

Thankfully, even after 100's of years, providing nothing else changes, vaccines and controlled exposure could be used to give the space dudes a fighting chance.


In the last few thousand years, AfroEuroAsian civilization experienced a large amount of urbanization and population density increases.

This resulted in diseases that cross over from other species to have a fertile ground to spread and infect in waves that cross and recross the world. Many millions if not billions die over the generations, those that survive adapt to being resistant to the kind of symptoms the disease generates, and the disease adapts to be more efficient at spreading.

When AfroEuroAsian explorers reached isolated communities who hadn't been exposed to the huge host of diseases they where used to, the diseases did a clean-field burn of the populations. Fewer diseases came back, as the volume (time times area) of dense human habitation wasn't as large; but possibly not absent. It is possible that syphilis did transfer back from the Americas.

The seed population of Astronauts would have a genetic pool from the surivors of such pandemics. They would also carry with them aquired immunity to common earthborn diseases; that, however, lasts at most a generation or two, and sometimes as little as a decade or less.

Their innate response to infection might not drift rapidly. That innate response is mostly what makes you die or not die to a given disease. A disease, say smallpox, might kill 1/3 of people who where culled by a previous smallpox epidemic in their genetic history, while kill 90% of those who lack that culling.

There would be some drift. Back on Earth, new diseases and variants would spread, and those whose innate immune response was fatal would die and tend not to pass on their genes. The red queen's race would continue.

Those on the station would have their innate immune response mostly drift, or respond to variations in the small number of diseases they carry with them. Immune systems that overreact to sterile environments with massive allergies might be selected against, for example. But if your immune response to the common cold (which dies out in a small population) in the next generation is massive and debilitiating, you wouldn't even notice.

There is also the possibility of a pandemic hitting Earth, killing a significant percentage of the population. Such a hit would result in those with an immune response that isn't fatal to that disease being selected-for. Multiple such pandemics could cause significant genetic drift over the entire Earth.

So these different factors would result in the innate immune response of the Space Fairing and Earth populations diverging. The Space Fairing would also experience the founder effect and larger amounts of drift due to smaller population size, where quirky unique mutations in the founders might end up being extremely common, or later mutations doing the same.

Over 7-10 years, your immunity to colds and flu would fade. Going back, you would be likely to get a really bad cold and possibly flu, which could be fatal. The next generation would get some antibodies from their mother via breast feeding, but their immune system would mostly be missing defences agianst common earth infections; so they would risk a really, really nasty lung/ear/eye/skin infections when they come back. This would increase over time. These can be fatal, but they aren't like 90% death rates with modern medicine.

The multi-generation effect of genetic drift and the red queen's race back on Earth could result in the isolated population having mass deaths. How long this would take is extremely hard to guess, as we haven't really had a good experiment. Even Pitcarn island was only isolated for 20-odd years.

Animal models aren't any good, because we are stange critters with our city-hives, worldwide travel, and quirky immune systems.


I think the first generation would already have serious problems. The immune system strengthens itself when it is in contact with foreign cells, when it returns to Earth without having any contact with these diseases, its immunity would be well below those who have always been in contact.


The immune system itself would not necessarily degrade over the generations in space. By design immune templates in the body are blank, what immunologists called "naive". When a person is born into a new biological environment, their body adapts to the local diseases. Breeding in space will not change this fundamental capability. A person who is born in space and suddenly returned to earth will be at risk from new pathogens, but this is no different than the risk you take any time that you travel. If you take a trip to Madagascar and have contact with the local people, you will be taking the same risk.

The only dynamic immunological information conferred between the generations comes to a baby from its mother either via placental transfer of immunoglobulin or at birth by nursing. When a mother first nurses she produces a substance called colostrum which encodes the local disease environment. By drinking the colostrum, a baby inherits many of the immunological templates of its mother. For this reason an astronaut's children or descendants could return to the same place on earth and have a degree of immunity to the local diseases in the area.

In general, a spaceship will be a different kind of environment than the earth because, as you point out, the pool of pathogens will be smaller. On the other hand, if a new pathogen were to evolve, it could potentially wipe out the space colony or greatly reduce its population.


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