The Causes

The Viruses. More than 200 different viruses are known to cause the symptoms of the common cold. Some, such as the rhinoviruses, seldom produce serious illnesses. 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 30 to 35 percent of all adult colds, and are most active in early fall, spring and summer. More than 110 distinct rhinovirus types have been identified. These agents grow best at temperatures of 33 degrees Celsius [about 91 degrees Fahrenheit (F)], the temperature of the human nasal mucosa.

Coronaviruses are believed to cause 10 to 20 percent of all adult colds. They induce colds primarily in the winter and early spring. Of the more than 30 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 10 to 15 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 causes of 30 to 50 percent of adult colds, presumed to be viral, remain unidentified.

The same viruses that produce colds in adults appear to cause colds in children. However, the relative importance of various viruses in pediatric colds is unclear because of the difficulty in isolating the precise cause of symptoms in studies of children with colds.

Does cold cause a cold? Although many people are convinced that a cold results from exposure to cold weather, or from getting chilled or overheated, NIAID grantees 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, and menstrual cycles may have an impact on a person's susceptibility to colds. For example, NIAID-funded experiments showed individuals under high levels of psychological stress are more prone to infection with any of five cold-producing viruses and more apt to experience respiratory symptoms than people experiencing less stress.

The Cold Season

In the United States, 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 may also 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.

Cold Symptoms

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 headache. Fever is usually slight but can climb to 102º F among infants and young children. Cold symptoms can last from two to 14 days, but two-thirds of people 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 a doctor's attention.

How Colds are Spread

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.

• Inhaling relatively large particles of respiratory secretions transported briefly in the air.

• Inhaling droplet nuclei: smaller infectious particles suspended in the air for long periods of time.

Research on rhinovirus transmission

Much of the research on the transmission of the common cold has been done with rhinoviruses, which are shed in the highest concentration in nasal secretions. Studies suggest a person is most likely to transmit rhinoviruses in the second to fourth day of infection, when the amount of virus in nasal secretion is highest. Researchers have also showed that using aspirin to treat colds increases the amount of virus shed in nasal secretions possibly making the cold sufferer more of a hazard to others.

NIAID grantees have found that rhinoviruses from nasal secretions can be transferred easily from the hands of an infected person to those of another--by shaking hands, for instance--or to a surface such as a doorknob or telephone that is then touched by another person. By touching one's eyes or nose with the fingers, something most people do many times a day, the susceptible person can be "self-inoculated." Other studies suggest rhinovirus colds can be transmitted through the air.

Preventing Transmission

Handwashing is the simplest and most effective way to keep from getting rhinovirus colds. Not touching the nose or eyes is another. Individuals with colds should always sneeze or cough into a facial tissue, and promptly throw it away. If possible, one should avoid close, prolonged exposure to persons who have colds.

Because rhinoviruses can survive up to three hours outside the nasal passages on inanimate objects and skin, cleaning environmental surfaces with a virus-killing disinfectant might help prevent spread of infection.

A cold vaccine? The development of a vaccine that could prevent the common cold has reached an impasse because of the discovery of many different cold viruses. Each virus carries its own specific antigens, substances that induce the formation of specific protective proteins (antibodies) produced by the body. Until ways are found to combine many viral antigens in one vaccine, or take advantage of the antigenic cross-relationships that exist, prospects for a vaccine are dim. Evidence that changes occur in common-cold virus antigens further complicate development of a vaccine. Such changes occur in some influenza antigens and make it necessary to alter the influenza vaccine each year.

Treatment

Only symptomatic treatment is available for uncomplicated cases of the common cold: bed rest, plenty of fluids, gargling with warm salt water, petroleum jelly for a raw nose, and aspirin or acetaminophen to relieve headache or fever.

A word of caution: several studies have linked the use of aspirin to the development of Reye's syndrome in children recovering from influenza or chickenpox. Reye's syndrome is a rare but serious illness that usually occurs in children between the ages of three and 12 years. It can affect all organs of the body, but most often injures the brain and liver. While most children who survive an episode of Reye's syndrome do not suffer any lasting consequences, the illness can lead to permanent brain damage or death. The American Academy of Pediatrics recommends children and teenagers not be given aspirin or any medications containing aspirin when they have any viral illness, particularly chickenpox or influenza. Many doctors recommend these medications be used for colds in adults only when headache or fever is present. However, researchers also have found aspirin and acetaminophen can suppress certain immune responses and increase nasal stuffiness in adults.

Nonprescription cold remedies, including decongestants and cough suppressants may relieve some cold symptoms but will not prevent, cure or even shorten the duration of illness. Moreover, most have some side effects, such as drowsiness, dizziness, insomnia or upset stomach, and should be taken with care.

Antihistamines generally don't relieve cold symptoms, because the body makes inflammatory chemicals other than histamine when attacked by a cold virus.

Antibiotics do not kill viruses. These prescription drugs should be used only for rare bacterial complications, such as sinusitis or ear infections, that can develop as secondary infections. The use of antibiotics "just in case" will not prevent secondary bacterial infections.

Does vitamin C have a role? Many people are convinced that taking large quantities of vitamin C will prevent colds or relieve symptoms. To test this theory, several large-scale, controlled studies involving children and adults have been conducted. To date, no conclusive data has shown that large doses of vitamin C prevent colds. The vitamin may reduce the severity or duration of symptoms, but definitive evidence is lacking.

Taking vitamin C over long periods of time in large amounts may be harmful. Too much vitamin C can cause severe diarrhea, a particular danger for elderly people and small children. In addition, too much vitamin C distorts results of tests commonly used to measure the amount of glucose in urine and blood. Combining oral anticoagulant drugs and excessive amounts of vitamin C can produce abnormal results in blood-clotting tests.

Inhaling steam also has been proposed as a treatment of colds on the assumption that increasing the temperature inside the nose inhibits rhinovirus replication. Recent studies found that this approach had no effect on the symptoms or amount of viral shedding in individuals with rhinovirus colds. However, steam may temporarily relieve symptoms of congestion associated with colds.

Prepared by:
Office of Communications
National Institute of Allergy and Infectious Diseases
National Institutes of Health
Bethesda, MD 20892
Public Health Service
U.S. Department of Health and Human Services