A simple common cold is a collection of familiar symptoms signaling an infection of the upper respiratory tract, which includes the nose, throat, and sinuses.

Colds are self-limiting diseases, meaning that their symptoms last a certain length of time (or "run their course") and then disappear without leaving lasting ill effects. A cold is a mild but commonplace disease, contracted by adults about two to four times a year and by children about six to eight times a year. Adults with children at home are more likely to catch colds than are those who do not have children.

Children are especially susceptible to colds because they have not yet developed immunity or resistance to the many viruses that can cause colds. Small children gradually build up immunity to the viruses in their homes, but when they go to school and have close contact with many other children, they are exposed to new viruses. Similarly, adults who travel frequently or have a high number of close contacts outside their community are more likely to contract colds or encounter new cold viruses.

Complications

A cold can be a minor irritation, but it can increase an individual's susceptibility to more serious conditions, especially in the very young, the very old, and the very weak. Pneumonia, an inflammation of the lungs, is probably the most serious of these conditions. Ear infections, sinus infections, and bronchitis are other possible complications. A few days after a cold, children sometimes develop croup, which is recognized by a harsh, barking cough that signals swelling of the airways to the lungs.

Causes

At least five major categories of viruses cause colds. One of these groups, the rhinoviruses, includes a minimum of 100 different types of viruses. Various combinations of symptoms and possible complications can develop from each of these viruses. It is not known exactly how viruses spread, but it seems to be a combination of physical contact and the presence of both virus particles and moisture in the air.

A virus can be spread by hand-to-hand contact, for example, or by the passage of droplets from an infected person's nasal passages and throat into the air. Colds have an incubation period of 48 to 72 hours, meaning that it takes that long after the virus enters the body for early symptoms to appear.

Symptoms

Early symptoms of the common cold include a stuffy or runny nose, sneezing, a sore or scratchy throat, a cough, and occasionally a mild fever. Usually, as the cold progresses, other symptoms -- burning or watery eyes, loss of the sense of taste or smell, pressure in the ears or sinuses, nasal voice, and tenderness around the nose -- may also appear.

Symptoms vary in type and severity from one viral infection to another, so a cold can begin with any symptom or combination of symptoms. Most colds last about a week, but about 25 percent of all colds last two weeks. Smokers and those with chronic respiratory diseases tend to have more severe symptoms and longer-lasting colds. They also experience complications more readily than do those who do not fall into these categories.

Diagnosis

Since common colds are mild diseases, the physician, in diagnosing a cold, will actually be looking for symptoms indicating a condition more serious than a common cold. Material from the patient's throat or nasal passages may be tested for bacterial infections. A blood test may be recommended to check for mononucleosis, a disease characterized by a long-lasting sore throat and swollen lymph nodes. An X-ray study of the sinuses may be necessary if sinusitis (an infection of the sinuses) is suspected.

Treatment

Getting plenty of rest, drinking lots of fluids to prevent dehydration, and using a humidifier or vaporizer can help relieve the irritating symptoms of a cold. Nevertheless, the common cold cannot be cured, and no known treatment will actually hasten recovery. Many over-the-counter (nonprescription) medicines and preparations are available that will at least ease the discomfort of a cold. However, it is best to take specific medications only for the symptoms actually present and to follow directions on the medication package carefully.

Overuse of an otherwise effective remedy can backfire and actually make the symptoms worse, and treating symptoms that are not there can complicate matters. For example, the use of a nasal decongestant for more than three days can actually increase congestion, because the blood vessels tire and relax, causing rebound congestion. Anyone who is pregnant or has a chronic disease should check with a doctor before using cold preparations, even seemingly harmless over-the-counter drugs.

Prevention

There is no known preventive for the common cold. Vitamin C has been said to help prevent colds, but many studies have shown that it has no measurable effect in this regard. Avoiding exposure to viruses, when possible, may be the only means of avoiding the common cold.