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Lets say you have a small group of space colonists. They were screened for sickness, viral and bacterial, as thoroughly as we knew how before being sent off to the colony and all were found healthy. They traveled for years in a fully sanitized ship before reaching their new colony, which is isolated from all other humanity, or other earth-animals. Effectively there is no one to catch a cold from on this new planet.

How long would it take for sickness and disease (viral and bacterial, not counting things like cancers) in this population? What length of time would it take for the equivalent of your common cold to pop up? How long would it take for more dangerous potentially life-threatening illnesses to crop up?

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    $\begingroup$ Are we to assume that all disease on this world is totally incompatible with humans? Because chances are high that if this planet has earth-like biology it has a virus or bacteria or parasite or prion or.... something else that will happily make us sick. $\endgroup$ – JDSweetBeat Oct 14 '15 at 19:12
  • $\begingroup$ @DJMethaneMan I'm assuming they are living in a world that did not have an earth like biom, because of hos absurdly unlikely it is for one to exist. Their teraforming or using domes to survive on otherwise inhospitable worlds $\endgroup$ – dsollen Oct 16 '15 at 14:27
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First of all, the colonists might become reactionary to some component of the equipment in their environment. Everything ages. Metals oxidize and flake, or react with other chemicals in the environment. Synthetics also break down due to heat, light, and mechanical stress. Second, any healthy human is going to carry their own viral, bacterial and fungal load. Mutation is a fact of life for these critters. I think the chances of one person developing a variant germ that affects the others are pretty high.

The human genome can also experience epigenetic changes due to the factors I mentioned.

I'd have to take a wild guess and say that symptoms would be noted in the first 5 years. For instance, a "cold" could be anything from a histaminic reaction to infection by someone else's mutated germ. Of course, your colonists would likely carry along some very sophisticated equipment and medical expertise.

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Every healthy human being consists of a huge number of microbes living inside him/her. More than a dozen of these microbes are those which cause diseases. As long as the immune system of the person stays strong, these microbes act as "good citizens" and stay in their limits. Once the immune system is compromised (malnutrition, blood loss, or any other weakness), these microbes start showing their dark side and get nasty.

So ... your space colonists have all the necessary ingredients for an epidemic within them. Once any of them gets weak, the microbes will get him/her sick and if it is a transmittable disease, soon an epidemic would break out.

If you are looking to learn about the speed of microbe evolution, here is a link that would help you. It is about an experiment designed just at what you are looking for: the speed of evolution in microbes.

E. Coli Long Term Evolution Experiment

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The length of time that the colony can go without getting sick will depend on the mutation rate of the bacteria/viruses/prions that the colonists brought with them. At mission start, the bacteria will be in homeostasis with the colonist's immune systems. (For simplicity, I'm going to refer to all infectious agents as bacteria.)

As bacteria breed, they mutate, picking up and dropping traits that increase or decrease their survival. If a mutation results in a strain of bacteria that a colonist's immune system can't handle without symptoms then the colonist will get sick. From there, the nature of the mutation will dictate how well the bacteria spreads and the exact symptoms of the illness.

Also, individual colonists may experience weaker immune systems as a result of stress, malnutrition or age which will make their bodies better breeding grounds for bacteria. Since there's a proportional relationship between the number of bacteria generations and the number of mutations, the longer a bacteria population survives, the more likely it is to have a mutation that will enable it to get a colonist sick.

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