Quick, in the last book you read, how many characters came down sick, and what length of time did the book cover? There are a lot of things that some authors just don't bother with when they are writing. Some never mention trips to the bathroom, or the clothes their characters wear, or the food they eat. If that works for them, for their stories, then that's okay, but sometimes I'm left wondering if a certain set of characters have any bodily functions, or have more than 1 outfit. Of course, I'd rather not have any author spend 6 pages going into great detail about each trip to the bathroom, or a description of today's outfit. I just like to know that I'm dealing with real people. People I can relate to.
People sometimes get sick. I'm not asking for anything life-threatening ... that would be too depressing, and I don't read to be depressed. But I don't know anybody who hasn't had an occasional cold, bout of flu, allergy attack ... something that makes them foggy-headed, out of sorts and functioning below par. For instance, so far this year, I have had a head cold and two bouts of stomach flu. Yes, it's been a bad year for me.
So, I suppose if a book covers only a few months, then it might not be too unbelievable that none of the main characters ever got sick. But when the span of that book is a couple years or more, I start feeling cheated if no one ever gets the sniffles! How can that many people be that healthy? It just doesn't seem believable.
An illness does not mean the action of the story has to halt until that person is healthy. How many times have you gone to work, even though you were mildly sick? Life does go on, which means the story can go on. SickPerson might make a mistake in a report that leads to complications. Or a foggy head might have a SickPerson phrasing a sentence poorly, leading others to think the wrong thing. Or, how about a taste of real reality? SickPerson's assignment has to be covered by Others.
Or, as I did in one of my stories, someone gets sick, but it hardly gets mentioned. In a book that spanned 2 years, a divorced mother spends a week fighting a cold that really has her down for the count. It is her landlady's efforts to care for his sick mother that has her 4-year-old son accept the landlady as someone he could trust. But I didn't show all of that; I simply had the son tell another adult that his mother had been sick, and Wanda (the landlady) had taken care of both of them.
Wait, was that one of those all-important trips to the bathroom that I should have devoted at least 6 pages to? Maybe. Maybe not. Every author sees these things differently.