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.
Symptoms of the common cold usually begin two to three days after infection and often include nasal discharge, obstruction of nasal breathing, swelling of the sinus membranes, sneezing, sore throat, cough, and headaches. Fever is usually slight but can climb to over one hundred degrees Fahrenheit in infants and young children. Cold symptoms can last from two to fourteen days, but up to two-thirds of patients recover in a week. If symptoms occur often or last much longer than two weeks, they may be the result of an allergy rather than a cold.
Colds occasionally can lead to secondary bacterial infections of the middle ear or sinuses, requiring treatment with antibiotics. High fever, significantly swollen glands, severe facial pain in the sinuses, and a cough that produces mucus may indicate a complication or more serious illness requiring medical attention. Viruses cause infections by overcoming the body’s complex defense system. The first line of defense is mucus produced by the membranes in the nose and throat. Mucus traps the material we inhale: pollen, dust, bacteria, and viruses. When a virus penetrates the mucus and enters a cell, it commandeers the protein-making system to manufacture new viruses which, in turn, attack surrounding cells.
Cold symptoms are probably the result of the immune response to the viral invasion. Virus-infected cells in the nose send out signals that recruit specialized white blood cells to the site of the infection. In turn, these cells emit a range of chemicals such as “kinins.” These chemicals probably lead to the symptoms of the common cold by causing swelling and inflammation of the nasal membranes, leakage of proteins and fluid from capillaries and lymphatic vessels, and the increased production of mucus.
Kinins and other chemicals released by immune system cells in the nasal membranes is the subject of intensive studies. Researchers are examining whether drugs to block them, or the receptors on cells to which they bind, might benefit people with colds.
Depending on the virus type, any or all of the following routes of transmission may be common: touching infectious respiratory secretions on skin and on environmental surfaces and then touching the eyes or nose. Also, inhaling relatively large particles of respiratory secretions transported briefly in the air.
By Colleen S. Mills, MD
Part 1 | Part 2
Photo courtesy of MedixNet