No one can as accurately describe the agonies of illness than those who are victims, and unless medicine has made you fuzzy-headed you could get some very good work out of being under the weather.
I'm am convinced that great writers throughout history have done this very thing. I always think of a line from Jane Austen's Emma when I have a cold: "There is nothing worse than a sore throat." It would not surprise me at all to find that when she wrote those words she was suffering from that very condition herself.*
Think for a moment of every time someone in a book you've read has been ill. Flu. Cold. Headache. All minor maladies could potentially have sprung from authorly under-the-weather-ness. Unless the health problem advances the plot, it may very well have been the authors projecting onto the characters. Maybe I'm reaching here but I find it an interesting, if unimportant, thought.
I must go and lie down and hope that my own illness is of short duration.
*There is a slight chance that this line is not in the book, but comes from a movie version of Emma. Should that be the case I must plead antihistamines as the cause of my mistake and promise to read Emma again soon so that I may avoid such embarrasing misquoting in the future.**
**I reserve the right to accidentally misquote other books,particularly when under the influence of cold medicines.