Found on active.com:
Here are some common sense tips for resuming workouts after being sick:
Don't rush it -- make sure you are FULLY recovered before you return to your exercise routines. If you have been sick for over a week, allow a few days of good health to pass before starting up again.
Avoid vigorous cardiovascular activities immediately after recovering from a cold or flu. Increased breathing may irritate and weaken your respiratory system.
Reduce intensity level and duration of your first few workouts after recovering from an illness. Do not attempt to make up for lost time by overdoing it.
Finish medications. Do not restart your exercise workouts until you have finished any prescription medicine or over-the-counter medications. Consult with your doctor if necessary.
Stay inside, especially in cold weather and avoid outdoor cardiovascular exercise. The increased stress on your body will weaken your immune system and lead to a relapse.
Stay warm. Hooded sweaters and fleeces are great for keeping your head and neck protected and your defenses up.
Listen to your body. If you are unusually tired, sense a return of any symptoms or just don't feel "right," ease off from your workouts and rest.
Don't forget your vitamins! Boost the immune system by increasing your vitamin C intake.
Don't panic that you have become out of condition because you weren't able to exercise while you were sick. Your health is much more important at this stage.