A lot of my readers probably come here from Transformetrics since I frequently post there. One question that I see a lot is what do when sick? Should you exercise when sick or should you skip it until you're better? There never seems to be a clear answer. I'm no expert but I definetly have some thoughts on the matter based on two things that I've read.
I recently read a book that described the intense training that goes on at Fairtex Muay Thai Kickboxing Acedamy in Thailand. The training and conditioning of this famous school is intense and extreme. The students train to the point where they often have to go to the hospital for simple infections and flu. The problem is that their intense training brings their bodies to a point where they're so overtrained that their immume system is compromised. This might seem like an extreme example but it does point out something important: You can exercise too much when you're sick. It does take a toll on your body and demands recovery when you need to direct your recovery powers to fighting off an infection or other illness.
Bed rest may not be the answer. I also read a survival guide before that states that if you have an ill or injured person in a survival situation that you must find something to keep them busy. Boredom and feelings of uselessness can kill someone as fast as their injury or illness can.
My thoughts on the subject of exercising while sick are this: If you feel good enough to do it, then some light exercise won't do you any harm and it might even help. I wouldn't do anything resembling a strenuous workout though. Just do enough to relieve boredom and get a nice endorphin rush. I do exercise when I'm sick. Sometimes, I'll just do some isometrics with my hands and forearms. Anything is better than nothing.
Then again, if you just can't move, then for goodness sake, listen to your body and take it easy. Don't exercise. You're not going to melt from three missed days. Listen to your body and make choices based on what your body is telling you what to do.