The answer is pilots SHOULD NOT fly with a cold. The reality is very different. Part of the problem is when is hay fever, which is common and manageable, actually a cold and progressively gets worse as the day goes on particularly on an 11.5 hour duty. The other part of the problem is when you go sick you forgo the flying credits so it costs you money to go sick. For most pilots that isn't a consideration but I have flown with crew who are in the early stages of cold/flu/URTI and that has been a motivation. Whether its right or wrong its the way it is.