What Is Influenza?

The flu, more scientifically known as influenza, is a highly contagious respiratory infection caused by influenza viruses. The influenza virus usually enters the body through mucous membranes in the mouth, nose, or eyes.

When a person with the flu coughs or sneezes, the virus then becomes airborne and can be inhaled by anyone nearby. You can also get the flu if you’ve touched a contaminated surface like a telephone or a doorknob and then touch your nose or mouth. Of course, the risk of infection is greater in highly populated areas like schools, buses, and crowded urban settings.

Who is at risk for flu complications?

Who is at risk for flu?

In most communities, school-age children are the first age group to get the flu. They then carry the virus home and to after-school activities where they interact with other kids. The flu virus is usually prominent from October through May, the time of year typically known as “flu season.”

Older adults, young children, and people with specific health conditions are at higher risk for serious flu complications. On average annually in the U.S.: 5% to 20% of the population gets the flu, over 200,000 people are hospitalized from flu complications, and between 3,000 and 49,000 people die from flu-related causes. Read more about the impact of the flu.

Is It a Cold or the Flu?

The cold and flu are both respiratory illnesses, but they are caused by different types of viruses with different symptoms. Use this chart to learn the differences between the two.

Signs and Symptoms Influenza Cold
Symptom onset Abrupt Gradual
Fever Usual; lasts 3-4 days Rare
Aches Usual; often severe Slight
Chills Fairly common Uncommon
Fatigue, weakness Usual Sometimes
Sneezing Sometimes Common
Stuffy nose Sometimes Common
Sore throat Sometimes Common
Chest discomfort, cough Common; can be severe Mild to moderate; hacking cough
Headache Common Rare

If you or a family member is experiencing symptoms that sound more like the flu than a cold, download our list of helpful Questions to Ask Your Doctor and call your doctor right away.

Types of Flu

There are 3 types of flu viruses. Type A viruses are found in many kinds of animals, including ducks, chickens, pigs, and also humans. The Type B virus widely circulates in humans. Type C has been found in humans, pigs, and dogs, and causes mild respiratory infections, but does not spark epidemics.

Influenza virus is one of the most changeable of viruses. Changes may be small and continuous or large and abrupt.

  • Influenza Type A is divided into subtypes that can be found worldwide and include H1N1, H1N2, and H3N2 viruses.
  • Influenza Type B outbreaks can also cause epidemics, but the illness it produces is usually milder than Type A.
  • Influenza Type C flu viruses are not thought to cause a large epidemic and generally cause only mild respiratory infections.

Flu viruses are constantly changing. A global flu pandemic (worldwide outbreak) can happen if 3 conditions are met:

  • A new subtype of Type A virus is introduced into the human population.
  • The virus causes serious illness in humans.
  • The virus can spread easily from person to person in a sustained manner.

Viruses vs Bacteria

Flu virus


A virus is a capsule of protein that contains genetic material. A virus cannot reproduce on its own and must infect a living cell to grow. Colds and influenza are both respiratory illnesses caused by different viruses — and it’s often hard to distinguish between the two.



Bacteria are independent, single-cell organisms that live on their own. They can multiply and reproduce by subdivision.


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