I guess I've been lucky in that I have never been sick at the same time I needed to fly. Can't imagine it would be very pleasant for anyone to fly with the flu, stomach ache, toothache, migraine, diarrhea, retinal detachment, emergency fungal toenail removal, etc.
What is the threshold for crew to call in sick?
When to put yourself on the no-fly list - Christopher Elliott- msnbc.com