When you come down with a cold or 'flu, you may be sufficiently wiped out that the very thought of exercising makes you shudder even worse. That's fine; exercising is about being healthy. The last thing you want to do is associate exercising with feeling even more awful than you already do after being trampled by a rhinovirus. Put the program on hold until you're on the mend and feel up to resuming it.
When you do start exercising again, you may be in for a surprise. If you stop exercising for four or five days, it's normal to have to drop back one rung and work back up. After a week of illness, you may have to drop three rungs or even more. People talk about a cold ``taking something out of you.'' With the feedback this program provides, you'll see it right in the numbers. What's more, you'll know when your recovery is complete, since that's when you'll return to the rung you'd attained before falling ill.
If you're seriously sick: typhoid, plague, creeping body rot, rely on the guidance of your doctor about exercise.