A character in the novel I'm writing -- a young woman -- has a condition/illness that can be in remission or without symptoms for long periods of time (i.e., years) but can then get worse, and then get better again. Basically a course that is unpredictable and that could potentially be serious, but could also, if she's lucky, not be. I was thinking of something autoimmune. Can you think of an illness that would fit this description?