Medical Health Encyclopedia

Colds and flus occur predominantly in the winter. Flu season typically starts in October and lasts into mid March. In 1999, for example, doctors' office visits significantly increased beginning in December and influenza activity peaked during the first 2 weeks in February.

The reasons for this seasonal bias are not due to the cold itself, but to other factors. Certainly, flus and colds are more like to be transmitted in winter because people spend more time indoors and are exposed to higher concentrations of airborne viruses. Dry winter weather also dries up nasal passages, making them more susceptible to viruses. Some experts theorize that the high rates of viral infections in winter may be due to certain immune factors, which react to light and dark and affect a person's susceptibility to viruses.

Traveling in Trains, Buses, and Planes

Traveling in close contact with people, whether on trains, planes, or buses, can increase the risk for respiratory infections. (A 2002 study suggested that the risk for a cold was about 20% after flying.) There has been particular concern that aircraft air that is recirculated can increase the risk for such infections. The same 2002 study, however, reported no difference in colds and flus among those who traveled in planes with fresh air versus recirculated air.

Day Care Centers

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Children who attend day care may have an increase risk of colds. However, a 2002 study suggested that although children in day care centers incur higher rates of the common cold in the preschool years, they have lower cold rates in their first years of regular school. The colds they catch in day care, then, may bestow some immunity to future colds for a few years. By age 13, such protection has worn off. There is also some evidence that frequent colds in young children may help protect against future allergies and asthma.