I believe we've talked about it before, but are you really supposed to exercise while sick? The conventional wisdom is that if the illness is above the neck, then go for it. If it is below the neck, lay low.
The idea to push ahead and go for it when you aren't feeling good can have different motivations. You get to avoid skipping workouts, to stay on schedule and not deviate from your routine. At the same time though, these workouts are usually at reduced intensity, so the overall benefit in terms of fitness gains are reduced.
According to the NIH, the current belief is that exercise increases the body's ability to fight off bacterial and viral matter, though the mechanisms aren't known. I would guess that increased blood-full and heart-rate would increase the overall efficiency of the circulatory system, which would in turn assist with efforts to fight off whatever bad bug you have. Also the sweat and waste produced from working out exits the system faster, carrying out the bad guys at the same time. But I'd love to see real studies on that.
In trying to find a shred of scientific literature about this (admittedly I didn't look to hard because I am in fact down for the count today), I came across this recommendation:
5 SECOND DIAGNOSIS
• sinus congestion
• runny nose and/or sneezing
• painful throat
Back to bed:
• temperature above 37.5 degrees Celsius, possibly interspersed with chills
• excessive tiredness not explained by activity level
• wheezing or tight chest
• nausea and/or vomiting
• body aches
I'd love to do a full-scale look into this further, but for now, I am going to exercise my right to continue sleeping on the couch while I get over this cold.