Plain and simple answer is:
Don't train while ill, especially with something like the flu. I often hear the excuse "but professional athletes compete when they are sick." While sometimes (not always) true, they are also paid millions of dollars to play, and have every reason to suck it up. Everyone else on the other hand... can usually afford to take a few days off from lifting without seeing a hit on the paycheck, which is more than you can say for yourself if your lifting prolongs your sickness or makes it worse, or your sickness leads to injury during exercise, both of which can keep you out of work.
Also considering the impact to recovery from illness and the illness' impact on recovery from exercise, there simply aren't enough pro's to warrant exercise while sick. A week (or two) at most isn't going to seriously unravel all your gains from your previous exercise regiment. At most you may have to backtrack a week or two in your overall exercise schedule. The most important thing to remember is that, once recovered from your illness, you get right back into your regiment in order to preserve your exercise habit.