The sense of smell depends upon the scented things in the air reaching the lining of our nose, especially certain small parts of the lining of the nose. When we have a cold, this lining, or mucous membrane, of the nose gets swollen, and produces a much greater amount of mucus than usual, as we all can tell by the number of handkerchiefs we have to use in a day. The chief reason why we can not smell so well when we have a cold is that this mucus constantly pouring out of the lining of the nose and running over it, prevents the scent of things getting to the sensitive part of the nose, and washes away any solid scented particles that there may be in the air. Also, it may very likely be that the poisons produced by the microbes that cause a cold, poison the living cells of the mucous membrane, and also poison the tiny ends of the nerves of smell that run to it, so that even if scented air does reach the sensitive part of the mucous membrane, they can not be felt.
This applies alike to scents coming in from outside and also to the scents of food, which pass up at the back of the roof of the mouth into the nose, and which, if we have not got a cold, help to give our food half its flavor.