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The Common Cold
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.
The Problem
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. In families with children in school, the number
of colds per child can be as high as 12 a year. Adults average about two to
four colds a year, although the range varies widely. Women, especially those
aged 20 to 30 years, have more colds than men, possibly because of their
closer contact with children. On average, individuals older than 60 have fewer
than one cold a year.
The economic impact of the common cold is
enormous. The National Center for Health Statistics (NCHS) estimates that, in
1994, 66 million cases of the common cold in the United States required
medical attention or resulted in restricted activity. In 1994, colds caused 24
million days of restricted activity and 20 million days lost from school,
according to NCHS.
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 a large
percentage 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. 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, 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 (throat), and menstrual cycles may have an impact on a person's
susceptibility to colds.
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 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.
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 in 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 Cold Viruses Cause Disease
Viruses cause infection by overcoming the
body's complex defense system. The body's 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 machinery to manufacture
new viruses which, in turn, attack surrounding cells.
Cold symptoms: the body fights back. Cold
symptoms are probably the result of the body's 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 immune system 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 lymph vessels, and the increased production of mucus.
Kinins and other chemicals released by immune
system cells in the nasal membranes are the subject of intensive research.
Researchers are examining whether drugs to block them, or the receptors on
cells to which they bind, might benefit people with colds.
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 secretions is
highest. Researchers also have shown 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.
Prevention
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 virus 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. Researchers, however, have found that 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.
Nonprescription antihistamines may have some
effect in relieving inflammatory responses such as runny nose and watery eyes
that are commonly associated with colds.
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 there is no definitive evidence.
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. But steam may temporarily relieve symptoms
of congestion associated with colds.
Interferon-alpha has been studied extensively
for the treatment of the common cold. Investigators have shown interferon,
given in daily doses by nasal spray, can prevent infection and illness.
Interferon, however, causes unacceptable side effects such as nosebleeds and
does not appear useful in treating established colds. Most cold researchers
are concentrating on other approaches to combatting cold viruses.
The Outlook
Thanks to basic research, scientists know more
about the rhinovirus than almost any other virus, and have powerful new tools
for developing antiviral drugs. Although the common cold may never be
uncommon, further investigations offer the hope of reducing the huge burden of
this universal problem.
NIAID, a component of the National Institutes of Health, supports research
on AIDS, tuberculosis, malaria and other infectious diseases, as well as
allergies and immunology.- Prepared by:
Office of Communications and Public Liaison
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
May 1998
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