When I start to get a cold, I feel generally quite ill for the first 24-36 hours, then feel ok again, aside from the specific cold symptoms.  I have always assumed that this turning point represented the point at which your immune system kicked in and started dealing with the virus.  But if this is the case, how come colds take such a long time to clear up?  Why do we go through a succession of symptoms (nose, throat, chest) -  if your immune system can recognise and deal with the virus when it's making your nose run, why doesn't it stop the progression to your throat and chest or at least deal with it a lot faster?  And, at what point do you stop being infectious?