Viral Influenza

Influenza, commonly called the flu, is always caused by a virus.

Influenza, also known as the flu, is caused by a virus.

There are different strains of influenza:

• Influenza A is the most widespread and most severe strain. It can affect both animals and humans. Influenza A is the cause of the worldwide epidemics (pandemics) of the flu that have occurred. An estimated 142,000 hospitalizations per year are due to this strain of the flu.  Influenza A is usually further categorized by two subtypes based on two substances that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N).
• Influenza B infects only humans. It is less common than Type A, but is often associated with specific outbreaks, such as in nursing homes. Flu caused by this strain tends to be milder than that caused by Influenza A.

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Based on a final analysis of the 2001-2002 flu season, nearly 90% were type A and about 10% were type B. Influenza A usually causes more severe disease than type B. However, because influenza B has been less common in the past few years, there is some concern that some people--particularly small children--may have fewer antibodies to it and so may be at higher risk for severe infection. (See Flu Vaccines in this report.)

Complications of the Flu. In general, the flu is usually self-limited and not serious. It is responsible, however, for 15 - 30% of the excess number of hospitalizations that occur in winter. About 1% of people who contract the flu end up in the hospital, and an estimated 36,000 people currently die each year of flu-related complications. The highest risks for serious complications occur in people age 65 and older and in those who are already sick with another disease. There have also been reports of flu-related deaths in very young children.