More than 200 different viruses are known to cause the symptoms of the common cold. 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 about 91 degrees Fahrenheit, the temperature inside the human nose.

Scientists think coronaviruses cause a large percentage of all adult colds. They bring on colds primarily in the winter and early spring. Of the more than 30 kinds, three or four infect humans. The importance of coronaviruses as a cause of colds 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, which cause flu), paramyxoviruses (including several parainfluenza viruses), respiratory syncytial virus, and enteroviruses.

There is no evidence that you can get a cold from exposure to cold weather or from getting chilled or overheated.

Cold symptoms can last from 2 to 14 days, but like most people, you'll probably recover in a week. If symptoms occur often or last much longer than 2 weeks, you might have an allergy rather than a cold.

Colds occasionally can lead to bacterial infections of your middle ear or sinuses, requiring treatment with antibiotics. High fever, significantly swollen glands, severe sinus pain, and a cough that produces mucus, may indicate a complication or more serious illness requiring a visit to your healthcare provider.

Nonprescription cold remedies, including decongestants and cough suppressants, may relieve some of your cold symptoms but will not prevent or even shorten the length of your cold. Moreover, because most of these medicines have some side effects, such as drowsiness, dizziness, insomnia, or upset stomach, you should take them with care. 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.