An upper respiratory infection is another way of saying that a person has a cold. A cold is a minor infection of the nose and throat. During normal breathing, air travels through the nose, down the trachea or windpipe, and into smaller and smaller airways. There are several mechanisms in place along the respiratory tract to prevent foreign particles and organisms from entering the nose and causing illness.
One mechanism involves mucus, which coats the inside of the nose and traps unwanted germs that might enter the airways. Cilia are tiny hairs that also help protect the body from germs by pushing out harmful particles that become trapped in the mucous lining.
If harmful germs slip by these defense mechanisms and wind up further down in the respiratory tract, they may enter the alveoli. It is here that the white blood cells come into play as they fight the unwanted organisms. At this point the classic symptoms of a cold develop: inflammation, swelling and irritation of the mucous membranes, sneezing, and sore throat.
Upper respiratory infections can be spread by direct contact, by exposure to airborne particles, or by touching contaminated surfaces and then by rubbing the nose or eyes.