I actually dug up some studies once upon a time. Exercise helps colds (you recover faster). It's not good for flu's. If I don't have a fever I usually ride.

It looks like at least one study recommends exercising with both a cold and a flu.
http://www.nlm.nih.gov/medlineplus/e...cle/002083.htm

http://www.nytimes.com/2008/12/25/he...on/25best.html