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Quick Facts

Influenza (Flu)


The Manual's Editorial Staff

Last full review/revision Oct 2021| Content last modified Oct 2021
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What is influenza?

What causes influenza?

The virus spreads:

  • Through the air, in droplets that an infected person spreads through a cough or sneeze

  • Through touching things that infected people have touched after wiping or blowing their nose

Flu season in the United States lasts from November through March. Each year, millions of people get the flu and thousands die.

The flu virus changes a little bit from year to year. These changes can cause problems:

  • Some changes make the virus more deadly

  • New vaccines have to be made each year

Sometimes the changes in the flu virus make it both more likely to spread and more deadly. This happens a few times every hundred years and causes huge worldwide flu epidemics. These epidemics kill millions of people around the world. Fortunately, because the virus keeps changing, this ultra-dangerous flu eventually changes to a less dangerous one.

What are the symptoms of flu?

Early symptoms of flu include:

  • Chills

  • Fever, up to 103° F (39.4° C)

  • Sick, weak, tired feeling

  • Muscle aches, especially in your back and legs

  • Severe headache

  • Runny nose

  • Scratchy, sore throat

Later symptoms include:

  • Severe cough that brings up phlegm (mucus)

  • Feeling sick to your stomach and throwing up

Most symptoms go away within a week, but the cough may last for weeks.

How can doctors tell if I have the flu?

Doctors can usually tell if you have flu based on your symptoms, especially if many people in your area have the flu. To know for sure, doctors may test your blood or fluid from your nose or throat for the flu virus.

If doctors think you may have pneumonia, they'll do:

  • Chest x-ray

  • Pulse oximetry, a test that uses a sensor placed on your finger to see how much oxygen is in your blood

How do doctors treat the flu?

Doctors will:

  • Tell you to get lots of rest and drink plenty of fluids, especially until your fever goes away

  • Tell you to take certain medicines to help you feel better, such as acetaminophen or ibuprofen for muscle aches and fever, or decongestants for a stuffy nose

  • Sometimes, give you antiviral medicines, if you’ve had symptoms for just 1 or 2 days

How can I prevent the flu?

Take these measures:

  • Get the flu shot

  • Wash your hands often with soap and water or hand sanitizer

  • Avoid crowds during flu season

  • Cover your nose and mouth when you cough or sneeze (if you have the flu)

  • Most flu shots are made using eggs, so people with severe egg allergies need to have a special egg-free flu shot—be sure to tell your doctor if you have an allergy to eggs

  • It takes 2 weeks for the flu vaccine to work, so doctors in the United States suggest getting it in October, before flu season starts

  • If you can't safely get a flu shot, doctors may give you antiviral medicine to prevent the flu if there’s an outbreak of flu in your area

Sometimes you still get the flu after you had a flu shot. However, the shot makes getting the flu a lot less likely. Also, many people who think they got the flu after having the shot really just had a bad cold and not the flu.

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