© (Getty Images) Avid exercisers might scoff at the idea of resting – even when they're sick. But sometimes, it's the smartest thing to do.

Tell fitness enthusiasts to sit still, and they'll scoff. Tell sick fitness enthusiasts to sit still, and their response will likely be the same. But should you really exercise when your body doesn't feel 100 percent? Medical professionals hear this question a lot and, with the flu season not too far away, it's a topic worth addressing.

Most clinicians recommend that if your symptoms mirror a common cold without a fever – in other words, the illness remains above the neck – you can exercise moderately by doing activities like walking. The American Council on Exercise supports low-impact physical activity of 30 minutes or less when exercising with a cold.

Exercising while sick may even be beneficial, since working up a sweat causes immune cells to circulate through the body more quickly and better kill bacteria and viruses. In a classicstudy published in the journal of Medicine & Science in Sports & Exercise, researchers recruited 45 18- to 29-year-olds of various fitness levels who agreed to be infected with rhinovirus, the culprit of most common colds. In two days, when symptoms reached their peak, the participants ran on treadmills at moderate and intense levels. The researchers reported that having a cold had no effect on participants' lung function or ability to exercise. Researchers also tested the runners at different points in the exercise sessions and found that their colds had no effect on the way their body responded to activity.

That said, these findings won't hold true with symptoms more severe than a common cold. With the flu or a more severe illness, when symptoms maybe include a fever, extreme tiredness, muscle aches or swollen lymph glands, you should rest and postpone intense exercise until at least two weeks after symptoms disappear. If you have doubts or think you may have a more serious illness, talk to your doctor.

Of course, the best case scenario would be avoiding sickness in the first place. Here are some tips to help ward off illness this flu season:

• Eat a well-balanced diet. The immune system depends on many nutrients, vitamins and minerals to do its best work. The best way to get these nutrients is through a healthy, balanced diet.
• Avoid rapid weight loss. Low-calorie diets, long-term fasting and rapid weight loss have been shown to impair immune function. In other words, losing large amounts of weight while training heavily can be taxing on the immune system.
• Get adequate sleep. Major sleep disruptions (think getting three hours less than normal) have been linked to immune suppression.
• Avoid overtraining and chronic fatigue. Space vigorous workouts and race events as far apart as possible. Listen to your body and don't push beyond your ability to recover.
• Manage your stress levels. Chronic stress has been linked to suppression of immune system function. Developing effective stress management strategies can go a long way in helping us stay healthy during the cold and flu season – especially in today's time-pressured and hectic environments. Exercise can help you feel less anxious, more relaxed and more confident. It can also encourage you to adopt other healthy behaviors. Activities such as yoga, meditation, deep-breathing and tai-chi are other documented ways to relax and reduce stress.

The bottom line: Use common sense when it comes to exercising during an illness. You should reduce the intensity and duration of your workouts by as much as 50 percent and listen to your body. If your symptoms worsen following your workouts, limit future workouts and rest until you get healthier. That will allow your immune system to recover.

It's important to remember that skipping a few workouts is not the end of the world. In fact, extra rest and recovery is sometimes the best thing you can do for yourself during an illness. And don't forget to be mindful of your fellow gym-goers: They don't want to get sick either.