It depends on you and to some extent how you feel. There have been times when I haven't missed an exercise session, and times when I skipped some because I couldn't get through them without coughing too much, and times when I couldn't get to the gym I felt so rotten. I try not to go if I know I'm contagious because I don't want to expose the rest of the world to my bug, but other than that, if I feel good enough to do the exercise I go. I usually cut my speed and inclination was back because I want to go for the time and endurance and I know I won't be able to do what I could do before I got sick.

During a flair-up ? are you kidding? you should be breating 2 in and 4 out through you puckered lips , telling yourself " be calm this will pass, breath ,breath , count 12, 1234 ) If you could do anything besides trying to breath you are not having a flair up.
If you are exercising when the flair up starts STOP!