I have missed a LOT of work over the last year because of CF. I had to go on a medical leave of absence and short-term disability. Fortunately, my supervisor was very understanding of my situation, and has been wonderfully accommodating now that I'm back at work. I've had to cut down to part-time (24 hrs/week) whereas before I was full time (40hrs). This has been a necessary step to preserving my health. My supervisor was also understanding about no longer including me in projects that would require travel. Although I'll miss that aspect of my job quite a bit, I know it's the right choice for me.
My company has an accrued leave policy, so that I accumulate time off at a rate based on how much time I work. I try to save those hours up like crazy so that when I need to dip into them, I have enough to get me through a rough patch.