If everything works fine, nobody will ever get sick on a space-ship. Most people will come aboard by transporter, which checks for (known) hazardous organisms. The food and water will usually be fine since it comes out of the replicator. There might be spaceborn viruses but if some of them enters the ship, there is probably a hull-breach or something (open shuttle-bays are protected by forcefield). The air should be well. Even if I don't know this for sure, Beverly mentions in one episode, that she can't infect someone due to air treatment in sickbay. It is conceivable that with that technology the air all over the ship is at least constantly monitored of harmful organisms.

All in all this does not mean that nobody will ever get sick. But it guarantees, that no-one will get sick if all works fine. If not (e.g. hull breach, transporter malfunction, unknown viruses, infected replicators, suspicious cargo), then someone might get a headache. So if someone sneezes, "doesn't feel so well", is inexplicably tiered, feels shaky, there is no way, that he can "just lay down and have a good nights sleep". He is to go to sickbay asap! All officers report to the bridge! All non-essential personal must stay in their quarters. We have a crises here, man!*

So why is everyone calm if someone gets sick as if that would be the most natural thing on a spaceship?

*Usually the plot agrees with my reasoning: If someone feels a little sick it usually means, that soon only Spock, Data, Odo or the Doctor will be able to perform any duty.

EDIT: To formulate the question more specific: Are there any harmless reasons to have e.g. a cold on a ship. And if so: Are those harmless incidences more often than the those that mean trouble? Since that would need to be the case if a responsible reaction to someone sneezing is sending him to his quarters.

FINAL EDIT Based on the reaction to sickness ("Go to your quarters" instead of "Red Alert!") it still must be verry common to get sick. So there are limits to the sickness-preventing technology. I wondered, what they are, where the weak links are located... I have no canon-based answer to that and appearently it is felt, that nobody has. So I withdraw this question.

  • This is very much opinion-based. The obvious answer is that we don't see the thousands of normal ailments that befall the crew, only the most extreme. – Valorum Apr 2 '14 at 15:06
  • Granted. But can there be a harmless reason to feel sick? If you are right, where does all the infectious microorganisms come from that befall the crew? Not from the food, not from away-missions... ? – Einer Apr 2 '14 at 15:09
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    @Einer Stupid question, but as you say "why is everyone calm if someone gets sick", did we ever see a crewman sneezing "just because they had a cold" in any of the Star Trek series? – Mr Lister Apr 2 '14 at 15:54
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    In current real life, people get colds all the time and we don't make a big deal out of it. In TV shows and movies, we probably see people with colds less often than in real life simply because that's not what the story is about (unless it is, and then it can become a big deal). The same thing could be happening in the Star Trek universe. If Ensigns Smith and Jones call in sick, it's probably just not going to show up on screen. – Keith Thompson Apr 2 '14 at 18:08
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    FWIW, you specifically mention "organism" detection, but viruses (such as the common cold or the flu) are not organisms. :) – Brian S Apr 2 '14 at 18:45

A "healthy" human body contains a large population of microorganisms, see: http://en.wikipedia.org/wiki/Human_microbiome

Some of these are beneficial for your health and well-being, such as intestinal flora which aid in digestion. Some may cause disease if their populations grow too large; or they may be perfectly safe for you, but not for someone else.

So, everyone on a starship is a walking reservoir of potentially pathogenic organisms and it would not necessarily be straightforward to detect them.

"Filtering the air" to get rid of these microorganisms won't be easy. What if two crew members are talking face to face? To prevent any air circulation between them, you would need something like a high-speed air curtain (basically, a very powerful fan) or a high-tech equivalent such as a forcefield. We could postulate that some sort of containment field operates around beds in sickbay, but it doesn't sound practical for every individual on the ship to be surrounded by an imperceptible filter bubble at all times.

It is true that if someone is shown sneezing in a Star Trek episode, the ship will quickly be overcome by a terrifying alien virus which can only be defeated by advanced handwaving, or possibly by learning to communicate and make friends. But we don't see all the occasions when a crew member developed a cold which turned out to be just a cold.

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    someone sneezing is a classic example of the Chekov's gun trope. – Valorum Apr 2 '14 at 15:13
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    Well, in "shades of gray" the transporter appears to be capable of detecting harmful organisms. I'm not sure that it can detect all organisms "that don't belong there". But if someone sneezes because of an organism undetectable to the transporter - wouldn't it be a reason to worry? – Einer Apr 2 '14 at 15:24
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    I agree on your air-filter explanation. But at least monitoring the air... Well if it's done it's not canonical anyway I think. So I withdraw the air-treatment-part from my question ;-) – Einer Apr 2 '14 at 15:27

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