I am sick with the typical cold/flu symptoms (second time in the past few months) and of course I feel like crap. I always want to work out more when I feel like this because it gives me some energy and temporarily makes me feel a little better. Is it REALLY that bad to exercise while you're sick? My doc says it's better to rest but I feel so much worse when I sit around and do nothing. But I don't want to prolong the sickness by taxing my body of the energy it needs to get better.

I asked that exact same question a couple of days ago.
I found that spending my usual amount of time but not putting full effort into it (except the stretches - I love those) really made me feel better without making me feel pooped.

If you are so unwell that you don't want to exercise, that's your answer. Let your body rest and heal. If, on the other hand, you just feel rather run-down but WANT to exercise, it is smart to have a lower-intensity workout, if you're going to have one.

It's pretty much common sense - I know a woman who will NEVER miss an hour of cardio - even when she had pneumonia. Stupid girl ended up in hospital, of course - if you have pneumonia, your lungs cannot do an hour of cardio. It is sensible to do as much or as little as you feel is best for you.

Get well soon!

EDIT: It takes a good ten days to two weeks for your cardio levels to drop, so taking a few days off to rest up when you aren't well is not only smart, but you don't have to worry about losing any ground on the fitness front!

Exercise boosts your immunity so I do think light workouts can help you get better faster when you are sick (see the article "Moderate Exercise Boosts Immunity" on About.com). I've definitely had some head colds where I continued to work out throughout the cold and I feel like the colds were milder or I was able to shake them off completely because of my workouts (of course, it's always hard to truly pinpoint causation in this situation--it could be that those colds would have been mild anyway).

The general rule is that if it's all above the neck and you feel up to it, go ahead and work out at a level you feel comfortable with. If the illness has moved down into your chest, you are running a fever, you are nauseous or experiencing digestive problems, you are achy, or you just don't feel like you have the energy for a workout, then you should rest. In any event, I wouldn't do a workout that is harder than what you would do normally; now isn't the time to push yourself. If anything, I would do a lighter workout. The article "Getting Through an Injury or Illness" on About.com also has some good tips about exercising when sick.

If I feel up to it, I work out. If I'm so tired I can barely motivate myself to get off the couch, then I stay put on the couch.