Sneezing, scratchy throat, runny nose everyone knows the first signs of a cold, probably the most common illness known. Although the common cold is usually mild, with symptoms lasting a week or less, it is a leading cause of doctor visits and of school and job absenteeism. In the course of a year, individuals in the United States suffer 1 billion colds, according to some estimates. Colds are most prevalent among children, and seem to be related to youngsters' relative lack of resistance to infection and to contacts with other children in day-care centers and schools. Children have about six to ten colds a year.
Most colds occur during the fall and winter.
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 also may affect the prevalence of colds. The most common cold-causing viruses survive better when humidity is low the colder months of the year. Cold weather also may make the nasal passages' lining drier and more vulnerable to viral infection.