Internally Peer Reviewed Beginning in late August or early September, the incidence of colds increases slowly for a few weeks and remains high until March or April, when it declines. The seasonal variation may relate to the opening of schools and to cold weather, which prompt people to spend more time indoors and increase the chances that viruses will spread from person to person. Seasonal changes in relative humidity may also affect the prevalence of colds. Cold weather also may make the nasal passages' lining drier and more vulnerable to viral infection.
Viruses cause infection by overcoming the body's complex defense system.
The body's first line of defense is mucus, produced by the membranes in the nose and throat. When a virus penetrates the mucus and enters a cell, it commandeers the protein-making machinery to manufacture new viruses which, in turn, attack surrounding cells. The University of Iowa. All rights reserved.