People don't tend to get colds a lot in SF/F. Unless you count that story of the man waking up in a future that has eliminated all disease, only to be euthanized when he says he has a cold. (It's been so long since I read it that I can't remember the author. Help me out, anyone?)
Probably colds seldom appear because the turning point of a plot doesn't tend to rest on whether Captain Whoever has the sniffles at the time. Most often in my reading I see plagues - serious ones - like the one in Anne McCaffrey's Moreta where the entire Pernese population comes under threat. Now that's a disease worthy of attention in a story!
This is not a bad thing. I've only read one series where people got colds, and were hurt every time they were hit by a rock or had a bad fall, a serious battle, etc. By the time I was 100 pages in it was so appalling that I was ready to laugh (or put the book down).
On the other hand, I do get concerned when I see people portrayed in abject poverty, having no visible means of health care or benefits of plumbing in their lives of squalor, yet who seem to have no illnesses. Parasites, anyone? Fevers, or malnutrition? And if not, why not?
I've actually spent a good deal of time agonizing over whether the poor in my Varin world should have good teeth. Cosmetic dentistry? Absolutely not. Orthodontia? Another no. But what about cavities and decay? Varin isn't a possible future earth, so tooth decay isn't exactly required. On the other hand, I don't think of Varin as a fantasy world, where it would be somewhat easier to imagine that mouth bacteria don't exist. And if it's science fiction, well... The jury's still out, but my current thought is that their technological level is high enough - the repressive government has probably fluoridated the water. Plausible, yet unobtrusive. With a high enough level of general societal technology, it's not too far out to imagine their medical profession is up to the task of getting that done.
As far as medical technology goes, there is a lot of room for flexibility. For low technology, you can look at the history of medicine in our world and vary it according to your needs. For high technology, you can extrapolate, which is always fun. Personalized medicine based on DNA is something people are already aiming at now, so I like to think it would be possible in the future.
Then there's medical culture. Who is the doctor? Is his or her role spiritual, scientific, both, neither? What of bedside manners? Is the doctor or the patient considered to know more about the nature of the complaint?
The last thing I want to mention is something I've been working with in a recent novel revision: How do you portray a population with weak immune systems, high incidence of mental illness, and high rate of infant mortality if none of your protagonists are currently sick? Yes, of course you can always say, "this population is inbred and has weak immune systems etc., etc." ...
How to describe the feeling I get from an explanation like that? Clunky, I could say. Or I could brandish my "show not tell" sword. But I think the best word for me is "external," or perhaps "distant." The explanation is something the author knows, but people living inside such a society would probably not be inclined to step back and talk about themselves that way (unless they were doctors making reports to the government, or something).
So instead I try to create a culture of health in the group I'm working with. I have them place labels on the sufferers - people "closeted" with deformities or chronic illness, or spoken of in whispers as "weak in the head." I make an increase in birth rate and infant health the stuff of public propaganda announcements. And I lace every slightest sign of ill health with a sense of fear in those who witness it. I have people make defensive statements about illness, always knowing internally that the diseases that cause minor affliction in others would likely kill them.
This actually forms a nice lead into my next topic, "worldbuilding in foreground vs. background." It's essentially "show not tell" - but I'm hoping to make it somewhat easier to think through, and to implement...
Upcoming posts at TTYU: worldbuilding in foreground vs. background; metaphor