I rarely ever catch a cold, norovirus or other common contagious diseases and always thought that I seem to have a good immune system. I do occasionally get a scratchy throat or a queasy feeling in my stomach for a couple of hours but that's about it.

However, a while ago I read that the symptoms of the common cold are caused by a person's immune response and not by the virus. So maybe my immune system isn't so good after all but actually rather lousy? BTW, I am also a smoker.