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Flu Learning Center

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What Is Influenza?

Influenza, commonly known as the flu, is a contagious respiratory disease caused by the influenza virus. The virus attacks cells in the upper respiratory tract, including those in the nose, throat, bronchi, and lungs.

The flu typically comes on hard and fast, and is characterized by high fever, chills, fatigue, body aches, sore throat, and a dry cough. These symptoms can be similar to the common cold, but the flu is caused by a different family of viruses, and symptoms are typically much more severe.

An infection with the flu virus generally runs its course in one to two weeks, but the flu sometimes leads to secondary infections or other complications, generally in certain high-risk groups including the very old and the very young. These secondary effects can cause serious illness or even death.

Flu virus can spread quickly through communities and areas where people work or live close together, leading to epidemics. Schools, nursing homes, and workplaces are at particular risk of outbreaks. Now and then an especially virulent form of influenza will infect people around the world in what is called a pandemic. The H1N1 virus in the 2009 to 2010 flu season was a pandemic. Flu epidemics and pandemics are most effectively prevented by widespread vaccination via an annual flu shot.

Types of Influenza

There are three main types of influenza virus that cause illness in human beings: Type A, Type B, and Type C.

Type A influenza virus is the form of influenza that causes the most severe illness, and is typically what people mean when they refer to the "flu virus." Type A influenza is common in the colder months of the year; its peak season ("flu season" usually runs from late fall to early spring. Type A influenza has historically been responsible for many flu pandemics over the years. Type B influenza is much milder than Type A viruses, but is active all year round. Type B influenza has also been linked to major outbreaks of the flu. Type C influenza is the least common type of the flu, and its symptoms are generally much milder than those of Type A or B influenza. It is generally believed that Type C does not cause epidemics.

Avoiding the Flu

For many people, vigorous personal hygiene may be enough to avoid the flu. For those at high risk, including children, seniors, those with weakened immune systems, and people with certain diseases, the flu vaccine is essential in providing protection against yearly flu outbreaks.

Each year's flu vaccine always includes the the two most active strains of Type A influenza. If public health officials determine that a new virus strain as emerged as a potential danger for that year, a special vaccine is made to combat the new strain of virus and is added to the annual vaccination.The strain of Type B virus most active in the population is usually included in the annual flu vaccine. Because it is rare and very mild, Type C is not included in yearly vaccines.

Content licensed from:

Written by: the Healthline Editorial Team
Medically reviewed by Jennifer Monti, MD, MPH

This feature is for informational purposes only and should not be used to replace the care and information received from your healthcare provider. Please consult a healthcare professional with any health concerns you may have.
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