What is the common cold?

Causes and Symptoms

One of the reasons that no cure has ever been found for the common cold is that it is caused by literally hundreds of different viruses. More than two hundred distinct strains from eight genera have been identified, and no doubt more will be discovered. Infection by one of these viruses may confer immunity to it, but there will still be scores of others to which that individual is not immune. The common cold is usually restricted to the nose and surrounding areas—hence its medical name, rhinitis (rhin meaning “nose” and itis meaning “inflammation”).

Children get the most colds, averaging six to eight per year until they are six years old. From that age, the number diminishes until, for adults, the rate is three to five colds per year. Colds and related
respiratory diseases are the largest single cause of missed workdays and school days. Colds and related respiratory diseases are probably the world’s most expensive illnesses. In the United States alone, about a million person-years are lost from work each year; this figure accounts for one-half of all absences. Worldwide, the costs of lost workdays, medications, physician’s visits, and complications that may require extensive medical care are incalculable.

Among the virus types that cause the common cold are rhinovirus, coronavirus, influenza virus, parainfluenza virus, enterovirus, adenovirus, respiratory syncytial virus, and coxsackie virus. They are not all equally responsible for cold infections.
Rhinoviruses and
coronaviruses between them are thought to cause 25 to 60 percent of all colds. Rhinoviruses appear to be responsible for colds that occur in the peak cold seasons of late spring and early fall. Coronaviruses appear to be responsible for colds that occur when rhinovirus is less active, such as in the late fall, winter, and early spring. Enteroviruses are the most common cause of the “summer cold.” During the summer months up to 20 percent of children may be shedding one of these viruses and thus are infective.

A respiratory syncytial virus can cause the common cold in adults; in children it causes much more severe diseases, including
pneumonia and bronchiolitis (inflammation of the bronchioles, small air passages in the lungs). Similarly, influenza and parainfluenza viruses, adenoviruses, and enteroviruses can be responsible for
rhinitis and
sore throat, but they are also capable of causing more serious illnesses such as pneumonia and meningitis.

Viruses are the smallest of the invading microorganisms that cause disease, so small that they are not visible using ordinary microscopes. They can be seen, however, with an electron microscope, and their presence in the body can be detected through various laboratory tests.

Viruses vary enormously in their size and structure. Some consist of three or four proteins with a core of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA); some have more than fifty proteins and other substances. Viruses can replicate only within living cells. They invade the body and produce disease conditions in different ways. Some travel through the body to find their target host cells. A good example is the measles virus, which enters through the mucous membranes of the nose, throat, and mouth and then finds its way to target tissues throughout the body. Some, such as the viruses that cause the common cold, enter the body through the nasal passages and settle directly into nearby cells.

Rhinoviruses are members of the Picornaviridae family (pico- from “piccolo,” meaning “very small”; rna from RNA, the genetic material that it contains; and viridae denoting a virus family). Coronaviruses are members of the Coronaviridae family, and they also contain RNA. Most viruses that are pathogenic to humans can thrive only at the temperature inside the human body, 37 degrees Celsius (98.6 degrees Fahrenheit). Rhinoviruses prefer the cooler temperatures found in the nasal passages, 33 to 34 degrees Celsius (91.4 to 93.2 degrees Fahrenheit). More than one hundred different rhinovirus types have been identified.

Exactly how a patient contracts a cold is better understood than it once was. Exposure to a cold environment—for example, getting a chill in winter weather—does not cause a cold unless the individual is exposed to the infecting virus at the same time. Fatigue or lack of sleep does not increase susceptibility to the cold virus, and even the direct exposure of nasal tissue to cold viruses does not guarantee infection.

A group in England, the Medical Research Council’s Common Cold Unit, studied the disease from 1945 to 1990 and made many fundamental discoveries—even though the researchers never found a cure, or, for that matter, any effective methods to prevent the spread of the disease. As part of their research, they put drops containing cold virus into the noses of volunteers. Only about one-third of the subjects thus inoculated developed cold symptoms, showing that direct exposure to the infecting agent does not necessarily bring on a cold.

What appears to be essential in the spread of the disease is bodily contact, particularly handshaking or touching. The infected individual wipes his or her nose or coughs into his or her hand, getting nasal secretions on the fingers. These infected secretions are then transferred to the hand of another person who, if susceptible, can become infected by bringing the hand up to the mouth or nose. Sneezing and coughing also spread the disease. Many viral and bacterial diseases are transmissible through nasopharyngeal (nose and throat) secretions; these include measles, mumps, rubella, pneumonia, influenza, and any number of other infections.

One or more individuals in a group become infected and bring the disease to a central place, such as a classroom, office, military base, or day care center. In the case of the common cold, transferring infected particles by touch exposes another person to the infection. In other respiratory diseases, breathing, sneezing, or coughing virus-laden particles into the air will spread the disease. The infected individual then becomes the means by which the disease is brought into the home. By far, the largest number of colds are brought into the family by children who have contracted the infection in classrooms or day care centers.

The pathogenesis of the common cold—that is, what happens when an individual is exposed to the cold virus—is not fully understood. It is believed that the virus enters the nasal passages and attaches itself to receptors on a cell of the nasal mucous membrane and then invades the cell. Viruses traveling freely in the blood or lymphatic system are subject to attack by white blood cells called phagocytes in what is part of the body’s nonspecific defense system against invading pathogens.

Once inside the host cell, the virus replicates itself by stealing elements of the protoplasm of the cell and using them to build new viruses under the direction of the RNA component. These new viruses are released by the host cell to infect other cells. This process can injure or kill the host cell, activating the body’s specific immune response system and starting the chain of events that will destroy the invading virus and create immunity to further infection from it.

In response to cell death or injury, certain chemicals are released that induce inflammation in the nasal passages. Blood vessels in the nasal area enlarge, increasing blood flow to the tissues and causing swelling. The openings in capillary walls enlarge and deliver lymphocytes, white blood cells that produce antibodies to fight the virus, as well as other specialized white blood cells.

Nasal mucosa swell and secretions increase, a condition medically known as rhinorrhea (-rrhea meaning “flowing,” denoting the runny nose of the common cold). During the first few days of infection, these secretions are thin and watery. As the disease progresses and white blood cells are drawn to the area, the secretions become thicker and more purulent, that is, filled with pus. A sore throat is common, as is laryngitis, or inflammation of the larynx or voice box. Fever is not a usual symptom of the common cold, but a cough will often develop as excess mucus or phlegm builds up in the lungs and windpipe.

As mucus accumulates and clogs nasal passages, the body attempts to expel it by sneezing. In this process, impulses from the nose travel to the brain’s “sneeze reflex center,” where sneezing is triggered to help clear nasal passages. Similarly, as phlegm accumulates in the windpipe and bronchial tree of the lungs, a message is sent to the “cough reflex center” of the brain, where coughing is initiated to expel the phlegm.

The common cold is self-limiting and usually resolves within five to ten days, but there can be complications in some cases. Patients who have asthma or chronic bronchitis frequently develop bronchoconstriction (narrowing of the air passages in the lungs) as a result of a common cold. If severe purulent tracheitis and bronchitis develop, there may be a concomitant bacterial infection. In some patients, the infection may spread to other organs, such as the ears, where an infection called otitis media can develop. Sinusitis, infection of the cavities in the bone of the skull surrounding the nose, is common. If the invading organism spreads to the lungs, bronchitis or pneumonia may develop.

Other possible complications of the common cold depend on the individual virus. Rhinoviruses, usually limited to colds, may infrequently cause pneumonia in children. Coronaviruses, also usually limited to colds, infrequently cause pneumonia and bronchiolitis. A respiratory syncytial virus causes pneumonia and bronchiolitis in children, the common cold in adults, and pneumonia in the elderly. Parainfluenza virus, which causes croup and other respiratory diseases in children, can cause sore throat and the common cold in adults and, rarely, may cause tracheobronchitis in these patients. Influenza B virus, an occasional cause of the common cold, also causes influenza and, infrequently, pneumonia.

Another condition that can closely resemble the common cold, but which is not caused by a virus, is
allergic rhinitis. The major form of allergic rhinitis is hay fever. It has many of the same symptoms as the common cold: sneezing, runny nose, nasal congestion, and, sometimes, sore throat. In addition, the hay fever victim may suffer from itching in the eyes, nose, mouth, and throat. Hay fever is an allergic reaction to certain pollens. Because the pollens that cause hay fever are abundant at certain times of the year, it may be prevalent at the same times as some colds. Spring is a peak season for the common cold and also for hay fever, because of the many tree pollens that are carried in the air. In the fall, weed pollens, such as those of ragweed, affect hay-fever sufferers during another peak period for colds. Colds occur less frequently in summer, but summer is another peak season for hay fever.

Treatment and Therapy

The nose is the first barrier of defense against the bacteria and viruses that cause upper respiratory infections. The nasal cavity is lined with a thin coating of mucus, a thick liquid that is constantly replenished by the mucous glands. Inner nasal surfaces are filled with tiny hairs, or cilia. Dust, bacteria, and other foreign matter are trapped by the mucus and moved by the cilia toward the nasopharynx to be expectorated or swallowed.

The blood vessels in the nasopharyngeal bed respond automatically to stimulation from the brain. Certain stimuli cause the vessels to constrict, widening air passages and at the same time reducing the flow of mucus. Other stimuli, such as those that are sent in response to a viral infection, allergen, or other irritant, cause blood vessels to dilate and increase the flow of mucus. Nasal passages become swollen, and airways are blocked.

The mucus-covered lining of the nasal passages contains various substances that help ward off infection and irritation by allergens. Lysozyme (lyso meaning “dissolution” and zyme from “enzyme,” a catalyst that promotes an activity) attacks the cell walls of certain bacteria, killing them. It also attacks pollen granules. Mucus also contains glycoproteins that temporarily inhibit the activity of viruses. Mucus has small amounts of the antibodies immunoglobulin IgA and IgC that also may inhibit the activity of invading viruses.

Bed rest is usually the first element of treatment for a common cold. Limiting physical stress may help keep the cold from worsening and may avoid secondary infections. The medications used to treat the common cold are directed at relieving individual symptoms; there is no available medication that will kill the viruses that cause it. Most cases of the common cold are treated at home with over-the-counter cold preparations. Children’s colds and the complications that may arise from colds, such as bacterial and viral superinfection, may require the services of a physician.

Many medications for the common cold contain
antihistamines. Histamine is a naturally occurring chemical in the body that is released in response to an allergen or an infection. It is a significant cause of the inflammation, swelling, and runny nose of hay fever. When these symptoms are seen with the common cold, however, they are probably caused by the body’s inflammatory defense system rather than by histamine.

When antihistamines were first discovered, it was thought that they could inhibit the inflammatory defense against a cold. Patients were advised to take antihistamines at the first sign of a cold, in the hope of avoiding a full infection. Current thinking is that antihistamines have little value in the treatment of the common cold. They may have a minor effect on a runny nose, but there are better agents for this purpose. Antihistamines are usually highly sedative—most over-the-counter sleeping pills are antihistamines—so they may cause drowsiness. Patients taking many antihistamines are cautioned to avoid driving or operating machinery that could be dangerous.

The mainstays of therapy for the common cold are the
decongestants that are applied topically (that is, directly to the mucous membranes in the nose) or taken orally. They are also called sympathomimetic agents because they mimic the effects of certain natural body chemicals that regulate many body processes. A group of these, called adrenergic stimulants, regulate vasoconstriction and vasodilation—in other words, they can narrow or widen blood vessels, respectively. Their vasoconstrictive capability is useful in managing the common cold, because it reduces the size of the blood vessels in the nose, reduces swelling and congestion, and inhibits excess secretion.

Topical decongestants are available as nasal sprays or drops. The sprays are squirted up into each nostril. The patient is usually advised to wait three to five minutes and then blow his or her nose to remove the mucus. If there is still congestion, the patient is advised to take another dose, allowing the medication to reach farther into the nasal cavity. Nose drops are taken by tilting the head back and squeezing the medication into the nostrils through the nose-dropper supplied with the medication. Clearance of nasal congestion is prompt, and the patient can breathe more easily. Nasal irritation is reduced, so there is less sneezing. Some nasal sprays and drops last longer than others, but none works around-the-clock, so applications must be repeated throughout the day.

Patients who use nasal sprays and drops are advised to follow the manufacturer’s directions exactly. Applied too often or in too great a quantity, these preparations can cause unwanted problems, such as rhinitis medicamentosa, or nasal inflammation caused by a medication (also called rebound congestion). As the vasoconstrictive effect of the drugs wears down, the blood vessels dilate, the area becomes swollen, and secretions increase. This reaction may be attributable to the fact that the drug’s vasoconstrictive effect has deprived the area of blood, and thus excited an increased inflammatory state, or it may simply be attributable to irritation by the drug. Use of sprays or drops should be limited to three or four days.

Oral decongestants are also effective in reducing swelling and relieving a runny nose, although they do not have as great a vasoconstrictive effect concentrated in the nasal area as sprays or drops. Because they circulate throughout the body, their vasoconstrictive effects may be seen in other vascular beds. Patients with high blood pressure; diabetes; heart disease; or who are taking certain drugs such as monoamine oxidase inhibitors (MAOIs), guanethidine, bethanidine, or debrisoquin sulfate are advised not to use oral decongestants unless they are under the care of a physician.

Three kinds of coughs may accompany colds: coughs that produce phlegm or mucus; hyperactive nagging coughs, which result from overstimulation of the cough reflex; and dry, unproductive coughs. If the phlegm or mucus collecting in the lungs is easily removed by occasional coughing, a soothing syrup, cough drop, or lozenge may be all that the patient requires. If the cough reflex center of the brain is overstimulated, there may be hyperactive or uncontrollable coughing, and a cough suppressant, such as dextromethorphan, may be needed. Dextromethorphan works in the brain to raise the level of stimulus that is required to trigger the cough reflex. Some antihistamines, such as diphenhydramine hydrochloride, are effective cough suppressants. If coughing is unproductive—that is, if the mucus has thickened and dried and is not easily removed—an expectorant should be taken. Currently, the only expectorant used in over-the-counter drugs is guaifenesin. It helps soften and liquefy mucus deposits, so that coughs become productive. When the cough of a cold is serious enough for a physician to be consulted, prescription drugs may need to be used, such as codeine to stop hyperactive coughing and potassium iodide for unproductive coughs.

For allergic rhinitis or hay fever, avoidance of allergens is recommended but is not always possible. For hay-fever outbreaks, antihistamines are the mainstays of therapy, with other agents added to relieve specific symptoms. For example, topical and oral decongestants may be required to relieve a runny nose.

Perspective and Prospects

Viruses are among the most intriguing and baffling challenges to medical science. Great progress has been made in preventing some virus diseases, such as by immunization against smallpox and hepatitis B. There has been only limited success, however, in finding agents to cure viral diseases, and so far nothing has been found to prevent or cure the common cold. Vaccines have been developed against certain rhinoviruses, and no doubt many more will be developed. Yet because the common cold is caused by so many different types of virus—more than two hundred—and vaccines against one virus are not necessarily effective against others, it is questionable whether such vaccines would ever be useful. A helpful vaccine would be one that could immunize against an entire family of viruses such as rhinoviruses or coronaviruses, the two leading causes of the common cold.

The search goes on for agents to cure the common cold. Substances, such as interferons, have been found that are effective against a wide range of viruses. One of the interferons was used by the British Medical Research Council’s Common Cold Unit. Those researchers reported that interferon applied as an intranasal spray was effective in protecting subjects from cold infection. After some years, however, experimentation with interferon in the common cold was abandoned because the agent had significant side effects, nasal congestion among them.

The science of virology only began in the 1930s, so it is not surprising that viruses continue to baffle scientists. Nevertheless, many fundamental discoveries have been made and one can predict increasing success. As scientists unravel the intricacies of viral infections, they find clues that help them devise ways of interfering with virus life processes. In some cases, effective drugs have been developed, such as the interferons, acyclovir for herpes simplex, and amantadine for the influenza virus. It is likely that the cure for the common cold will continue to be elusive, unless a broad-spectrum antiviral agent could be developed that works against multiple viral infections in the way that broad-spectrum antibiotics work against multiple bacterial infections.

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