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 one or two days to about a week, it is a leading cause of doctor visits and of school and job absenteeism. In the course of a year, millions of individuals in the United States would experience a cold. Colds are most prevalent among children, and seem to be related to their relative lack of resistance to infection and to contacts with other kids in day-care centers and schools. Children have about six to ten colds a year; in families with children in school, the number of colds per child can be as high as ten or twelve a year.
Adults average about two to four colds a year, although the range varies widely. Women between the age of twenty and thirty seem to have more colds than men, possibly because of their closer contact with children. On average, individuals older than sixty have less than one cold a year. The economic impact of the common cold could be important; it has been estimated that in 2007 over 65 million cases of the common cold in the United States required medical attention or resulted in restricted activity.
More than two hundred types of viruses are known to cause the common cold. Some, such as the rhinoviruses, seldom produce a serious illness. Others, such as parainfluenza and respiratory syncytial virus, produce mild infections in adults but can precipitate severe lower respiratory infections in young children.
Rhinoviruses, from the Greek “rhin,” meaning nose, cause an estimated 35 percent of all adult colds, and are most active in early fall, spring and summer. More than one hundred types of rhinovirus have been identified; they grow best at temperatures of about 90 degrees Fahrenheit, the temperature of the human nasal mucosa.
Coronaviruses are believed to cause a large percentage of all adult colds. They induce colds primarily in the winter and early spring. Of the more than thirty isolated strains, three or four infect humans. The importance of coronaviruses as causative agents is hard to assess because, unlike rhinoviruses, they are difficult to grow in the laboratory. Approximately fifteen percent of adult colds are caused by viruses also responsible for other, more severe illnesses: adenoviruses, coxsackieviruses, echoviruses, orthomyxoviruses (including influenza A and B viruses), paramyxoviruses (including several parainfluenza viruses), respiratory syncytial virus, and enteroviruses.
The cause of about half of all adult colds, thought to be viral, remains unknown. The same viruses that produce colds in adults appear to cause colds in children. The relative importance of various viruses in pediatric colds, however, is unclear because of the difficulty in isolating the precise cause of symptoms in studies of children with colds.
Does cold weather cause a cold? Although many people are convinced that a cold results from exposure to cold weather, or from getting chilled or overheated, several studies have found that these conditions have little or no effect on the development or severity of a cold. Nor is susceptibility apparently related to factors such as exercise, diet, or enlarged tonsils or adenoids. On the other hand, research suggests that psychological stress, allergic disorders affecting the nasal passages or pharynx (throat), and menstrual cycles may have an impact on our susceptibility to colds.
In the U.S., 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, during the colder months of the year. Cold weather also may make the nasal passages’ lining drier and more vulnerable to viral infection.
