I have an illness-bad dream. I recognice it by now, it's the same nightmare I've had since I've been sick as a child... So whenever I'm seriously ill, I get that dream... And when I see that dream, I know that there's seriously something wrong with my health. It's a kind of... purgatory-sort of thing. Hard to explain.

It's a helpful thing for a book, when the character has that illness-dream, because you get familiar with it and you can actually proceed in it (well, I can't, mine is a looping emotion thing, that still makes me flinch whenever someone whispers straight into my ear...)

It's been used in a few books too, Sillanpaa's Nobel-winning book about a girl with tuberculosis has those illness-dream sequences, and another Finnish sci-fi-ish book uses it too. I was pretty surprised about that.