I'll try this again, since my last post didn't seem to 'show up' even though my name did on the main page. Anyway...
Don't start working out until you can breath fully, deeply and without discomfort or congestion. This can be one to three weeks. (In other words, if you still feel like your 'getting over it' then your not ready.) If you start working out before this your likely to have a relapse. This I can relay from experience (twice - since I thought the second time that the first was an anomaly.)
You will lose some muscle mass, but once you start back at the gym, you'll get it back in a few weeks if you work up to your previous levels over a two week period. When you start up again, start with aerobic activity at about half of where you were previously, and weights at about 70% of where you were previously.
As for eating, you need to cut back in proportion to your workouts. If your workouts took about 12 hours a week, then cut back by at least a half. Eat really healthy and balanced.
Rest as much as you can and drink a lot of water. Cut back on caffeine if you drink a lot of coffee.