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If a human colony is set up on another planet or moon, or in an orbiting habitat, most of the infectious diseases from Earth would not be present in the small initial population. Careful screening could further reduce the number of infectious diseases introduced. Would the human immune system suffer problems if infection is reduced too much? Would we need to take some token infections (such as the cold) with us?

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This is difficult to answer definitively. Certainly, humans require certain microorganisms in order to be healthy, and the effect of carrying around such microorganisms is that with their shorter generations, the probability of a mutation making the organism pathogenic is small but significant.

However, it is difficult to say if a lack of pathogenic organisms is deleterious. As an undergraduate student of human physiology, I have developed the hypothesis - as yet without any proof either supporting or disproving it - that the human/mammalian immune system has evolved to expect a certain level of exposure to pathogens which would require an immune response, and if such a level of exposure is not achieved, the likelihood of autoimmune diseases is elevated.

Hence, our habit of sterilizing everything our kids come into contact with could well have the effect of setting them up for an autoimmune disease later in life. To not have any pathogens at all? That could be disastrous, or not.

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    $\begingroup$ The auto-immune response theory is well established with a lot of credibility, I've heard it from a number of different sources. I'm not sure what (if any) attempts have been made to prove/disprove the theory though. $\endgroup$ – Tim B Sep 21 '14 at 19:19
  • $\begingroup$ Our overuse of antibiotics in the mid/late-20th century will in retrospect turn out to have been a test of this hypothesis (though it has the disadvantage that it also wrecks the non-pathological gut microbiome). $\endgroup$ – octern Sep 21 '14 at 21:31
  • $\begingroup$ @octern no it won't. There's no control group and too many confounding factors, for instance, the obvious one, of more resilient bacteria. $\endgroup$ – djechlin Oct 24 '14 at 21:56
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    $\begingroup$ Also, over-use of antibiotics doesn't prevent exposure of the immune system to pathogens, or even to bacterial pathogens. You don't even know you're sick and start taking the antibiotics until there's already an immune response. Overuse of antibiotics might test something, but since it hasn't eliminated pathogens the thing it's testing isn't "no exposure to pathogens". I'm not certain it even results in reduced exposure to pathogens, except some chronic infections that used to be ubiquitous but now get cleared up. e.g. maybe TB is now less prevalent than 100 years ago? $\endgroup$ – Steve Jessop Feb 17 '15 at 9:11

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