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influenza or flu, acute, highly contagious disease caused by any one of at least three types of virus, but usually by types designated A and B. Formerly known as the grippe, influenza is difficult to diagnose in the absence of an epidemic, since it resembles many common respiratory ailments. It can be distinguished from a cold, however, by sudden fever, prostration, weakness, and sometimes severe muscular aches and pains. It is spread by respiratory droplets.
Influenza is usually self-limiting, but complications such as pneumonia and bronchitis can be serious threats to newborns, the elderly, and people with chronic diseases. Epidemics of influenza (usually of type A), sometimes worldwide in scope, have decimated large populations (more than 20 million people in 1918). Type B is more likely to occur sporadically.
Because a number of different viral strains can cause the disease, immunity to one type does not prevent susceptibility to another. Influenza virus vaccine confers immunity to a particular strain and is recommended for persons at risk; unfortunately the surface antigens of flu viruses change periodically, making it necessary to reformulate the vaccine yearly in an educated guess at what strain will appear.
The antiviral drugs amantadine and rimantadine are effective against type A influenza, and zanamivir against types A and B. Treatment with antibiotics has greatly reduced fatalities from secondary infections. Uncomplicated influenza requires only rest and treatment of symptoms. Return to normal activity should be undertaken slowly, as relapses are easily precipitated.
See G. Kolata, Flu (1999).
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