Learn About Flu

Influenza (flu), also referred to as seasonal flu, is a highly contagious respiratory illness caused by influenza viruses.

Key Facts

  • Anyone can get the flu as it is spread easily from person to person, usually when an infected person coughs or sneezes.
  • It's caused by one of three different viruses, although most serious illness is caused by flu strains A and B.
  • If you have asthma or other lung diseases, you are at higher risk of developing complications from the flu.
  • Getting vaccinated every year is the best way to protect yourself from the flu.

What Is Influenza?

The flu is a respiratory infection caused by flu viruses. The viruses pass through the air and enter your body through your nose or mouth. It can also be acquired from contaminated surfaces. Between 5 percent and 20 percent of people in the United States get the flu each year. It can cause mild to severe illness, and in some cases, may lead to hospitalization or death. Symptoms of the flu are similar to those of the common cold, but they last longer and tend to be worse.

Types of Influenza

There are three different influenza virus families: A, B and C.

  • Influenza Type A viruses can infect people, as well as birds, pigs, horses, and other animals. There are different strains (or subtypes) of influenza type A viruses, two of which circulate among humans: H1N1 and H3N2. These two subtypes are included in the seasonal flu vaccine each year.
  • Influenza Type B viruses are usually found only in humans. Influenza B viruses can cause illness among humans, but in general, are associated with less severe infection than influenza A viruses.
  • Influenza Type C viruses cause mild illness in humans. Influenza C cases occur much less frequently than A and B and are not included in the seasonal flu vaccine.

Influenza types A and B are the most severe of the flu viruses. The viruses change constantly and different strains circulate around the world every year. The body's natural defenses cannot keep up with these changes. Type C causes either a very mild illness or has no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do.

In addition to seasonal flu, there is another type of flu virus:

  • Avian flu (H5N1): Avian influenza, or bird flu, is also a subtype of influenza type A viruses. While highly contagious in birds, it does not usually infect humans. The virus is mainly transmitted to humans by direct contact with infected live, sick or dead poultry. However, it is thought that a few cases of human-to-human spread have occurred.

How Influenza Affects Your Body

The flu affects your whole body—including your nose, throat and lungs—and can lead to serious complications in those with chronic illness.

Many people who become sick with the flu say it is like being hit by a truck. Flu symptoms, such as high fever, cough and muscle aches, usually come on suddenly and are more severe than colds. Not only does having the flu impact your daily activities, it also leads to missed days from work and school.

Complications can include pneumonia, ear or sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes.

The flu can be fatal in elderly people, people with chronic diseases; and anyone with a weak immune system.

Learn more about the connection between the flu and pneumonia.

How Serious Is Influenza?

This decrease in quality of life, the impact of symptoms, the shifting nature of the virus, and the danger of life-threatening complications, combine to make the flu a major public health problem. 

Each year between 3,000 and 49,000 Americans die from flu and its complications. Although most people are back on their feet within a week after having the flu, certain people are more susceptible to complications. If you have asthma or other lung diseases, or other chronic conditions you are at higher risk of developing complications from the flu. Learn more about risk factors.

Prevent the Flu: Get Vaccinated

Everyone 6 months of age and older should get a flu vaccination every year. Most likely, this includes you and your entire family. Speak with your healthcare provider to learn more about influenza and annual vaccination. Find a flu vaccine provider near you.

Reviewed and approved by the American Lung Association Scientific and Medical Editorial Review Panel.

Page last updated: October 19, 2023

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