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Both colds and flu are caused by a virus, but they're not the same illness. The common cold can come from one or more of over 200 different viruses that can cause an infection once they invade the body. Influenza, or the flu, is caused by multiple strains of two types of a single family of viruses that change their characteristics every year. The types are called Type A and Type B. (There is a third type, C, that rarely causes disease in humans.) Different strains appear every year, so that one year's immunity is not completely effective the next year.
Colds cause sore throat, runny nose, nasal congestion, low-grade fever with muscle aches and a general sick feeling. They may also cause laryngitis. Flu symptoms include a high fever, dry cough, much worse aching all over and feeling terrible. Colds last about a week and the flu lasts as long as two weeks. Colds are more common during the winter, but the flu appears almost only during the winter months in the northern hemisphere.
Both cold and flu viruses enter the body through the nose and mouth, where they start to invade the sinuses, throat and upper breathing tubes (trachea and bronchi). Most healthy people can recover from colds and flu on their own, and develop some degree of immunity to future infections in the process. The flu, however, can be very serious for the elderly. A doctor should check older patients carefully if the flu is suspected.
This article was reviewed and updated June 2007.
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