Influenza (Flu)


What is influenza (flu)?

The flu is a respiratory infection caused by a virus (germ). Influenza occurs most often during the winter and easily spreads from person to person. The "flu season" in the Northern hemisphere runs from October to May and usually peaks between December and February.

Most people who get influenza feel sick for a week or two and recover. In some people, the flu leads to more serious lung infections or to worsening of underlying conditions, such as heart failure or emphysema.

Symptoms and Causes

What are the symptoms of influenza (flu)?

  • Sudden onset of moderate to high fever.
  • Dry cough.
  • Headache.
  • Sore throat.
  • Chills.
  • Runny nose.
  • Loss of appetite.
  • Muscle aches.
  • Tiredness.

Many conditions — such as a common cold, diarrhea, and vomiting — are called "the flu," but are not really influenza. “Stomach flu” is a misnomer, since viruses other than the flu cause such illness.

How can you tell the difference between a common cold and influenza (flu)?

Many cold and flu symptoms are similar. Both the common cold and the flu are caused by viruses.

There are some differences with influenza. Symptoms of influenza often hit suddenly and cause you to become weaker and weaker. While the more uncomfortable symptoms of flu generally last from three to seven days, the dry cough and fatigue of influenza can last two to three weeks. Signs that influenza is getting worse include increasing degree of fever, and shortness of breath. If you think that your illness is getting worse, contact your doctor right away.

Symptoms of colds and flu

FeverAdults-rare; children-sometimesHigh fever (100°F up to 104°F in adults and 106°F in children); can last 3 to 4 days)
Runny noseCommon (Nasal discharge may have a yellow- or green-colored tint. This does not mean you have a bacterial infection.)Sometimes
Stuffy noseCommonSometimes
HeadacheSometimes (usually mild)Common (usually severe)
Body aches and painsSometimes (usually mild)Common (can be severe)
Fatigue (tiredness), weaknessSometimes (usually mild)Common (can last up to 2-3 weeks)
Chills, sweatingNot commonCommon
NauseaUncommonCommon in children
Loss of appetiteSometimesCommon
CoughCommon (mild to moderate)Common (can be intense, severe)
Sore throatCommonSometimes
Chest congestion, discomfortCommon (mild to moderate)Common (can be severe)
VomitingNot commonSometimes (more in children)
DiarrheaNot commonSometimes (more in children)
Watery eyesCommonSometimes
ComplicationsSinus congestion, earacheBronchitis, pneumonia (can be life-threatening)

Diagnosis and Tests

When should you contact your doctor about influenza (flu)?

It’s important to take flu medicines very early (within 48 hours of coming down with symptoms), so you should contact your doctor as soon as you think you may have the flu.

Management and Treatment

How is influenza (flu) treated?

Most people with influenza who are otherwise healthy do not need special drugs or treatments. If you have the flu, you should:

  • Rest.
  • Drink lots of fluids.
  • Eat a light diet.
  • Stay at home.
  • Take acetaminophen (such as Tylenol®) to reduce fever and relieve muscle aches.

Note: Adults should not give aspirin to children or adolescents with fevers due to the association with Reye's syndrome, a rare disorder that causes brain and liver damage.

Can you get medicine for influenza (flu)?

If you are seriously ill, your doctor might order an antiviral drug for you. Antiviral drugs for influenza include oseltamivir phosphate (Tamiflu®); zanamivir (Relenza®); peramivir (Rapivap®); and baloxavir (Xofluza®).

Oseltamivir phosphate

This drug is approved to treat influenza in patients who are two weeks of age and older, and it works best in people who have had the flu for fewer than two days. It is also approved to prevent flu in patients who are one year of age and older. There is a generic version of this product available, but it costs nearly as much as the brand name. Potential side effects include nausea, vomiting, nosebleeds, headaches and tiredness.


This drug is approved to treat flu in patients seven years old and older, and to prevent flu in patients who are five and older. This product is inhaled and not recommended for people who have respiratory illnesses like COPD or asthma. Common side effects include headaches, nausea, diarrhea, nose irritation and vomiting.


This drug is approved to treat flu in people 2 years old and older. This product is given into the vein (intravenously) by a healthcare provider. A common side effect from peramivir is diarrhea.


This drug, a pill, is approved to treat flu in people 12 years old and older who are otherwise healthy and in people who are at higher risk of developing influenza-related complications. Common side effects are diarrhea, bronchitis, nausea and headaches.

The side effects mentioned for the above drugs are only the most common. There are other possible side effects. As with any type of medication, you might be allergic. Please discuss side effects with your doctor or pharmacist.

Amantadine and rimantidine have also been approved to treat influenza, but flu viruses are widely resistant to them.

What complications are associated with influenza (flu)?

Infections from bacteria are more likely when you have influenza. Healthcare providers treat these bacterial infections with antibiotic drugs. Common secondary infections include:


Can you prevent influenza (flu)?

Yes. If you get the flu vaccine, you are likely to be protected from the flu for the duration of the flu season. The vaccine is given as a shot or a nasal spray. You must get the vaccine every year in the fall to be protected. Sometimes the vaccine does not prevent you from getting the flu but makes the flu less severe if you do get it. The vaccine is safe, even for pregnant women. You can’t get the flu from the ‘flu shot.’

In addition, some of the antivirals (Relenza and Tamiflu) given to treat flu can be given to prevent flu in people who are in close contact with people who actually have the flu.

Because the flu is so contagious, you can do other things that may help you prevent getting or spreading the flu:

  • Practice good hand-washing hygiene. Wash your hands thoroughly with soap and water. If you aren’t able to use soap and water, use an alcohol-based hand sanitizer.
  • Avoid being around other people when you do not feel well, especially when you have a fever.
  • Avoid being around sick people whenever possible.
  • Avoid touching your eyes, nose and mouth.
  • Eat well, exercise, and get enough rest.
  • Consider taking a multivitamin and possibly vitamin D supplements to support your immune system. (Ask your healthcare provider if they think you need extra D.)

Who should get the flu vaccine?

It’s recommended that everyone 6 months or older should get an influenza vaccine each year. You will protect yourself and other people around you. People who have any of the following conditions are at high risk of becoming seriously ill from influenza :

  • Lung disease.
  • Kidney disease.
  • Liver disease.
  • Neurologic diseases.
  • Diabetes.
  • Heart problems.
  • An illness that weakens the immune system, or if you are taking a medicine that weakens the immune system, thus making it hard for your body to fight illnesses.
  • Blood disorders.
  • Obesity

You also have a higher risk of becoming seriously ill from influenza if you:

  • Are younger than 2 years, or over 65 years old.
  • Are pregnant and for 2 weeks after delivery
  • Are under 19 years old and must take aspirin regularly.
  • Live in a nursing home.

If you work in a healthcare facility, you may transmit influenza to patients and other workers, but you are not at a higher risk of becoming seriously ill. The recommendation from the Centers for Disease Control and Prevention is that everyone over the age of 6 months gets a flu vaccine if there are no contraindications. This includes individuals who are not at high risk.

Who shouldn’t get the flu vaccine?

You shouldn’t get the influenza shot if you are

  • Severely allergic to a previous dose of any influenza vaccine, regardless of the vaccine component (so including eggs) suspected of being responsible for the reaction.
  • Sick with a fever. (Wait until you are better.)

There is an option to get the nasal flu vaccine (administered through your nose). The following groups of people shouldn't get the nasal flu vaccine:

  • Children and adolescents who are taking aspirin or any type of salicylate-containing medication therapies.
  • Children who are 2-4 years of age who have been diagnosed with asthma or whose parents/caregivers can say that a healthcare provider has told them during the past 12 months that the child has had wheezing events or asthma; or a child who has a wheezing episode documented in their medical record.
  • Children or adults whose immune systems are compromised for any reason, including drugs or HIV infection.
  • Caregivers or close contacts of severely immunosuppressed people who need a protected environment.
  • Pregnant people.
  • People who have received antiviral drugs to treat the flu within the past 48 hours.

(Please remember that the above list is for people who should not receive the NASAL flu vaccine. It does not refer to the flu shot.)

When should you get the flu vaccine?

The best time to get the flu vaccine is in the early fall. It takes about 3 weeks for the vaccine to exert its protective benefits, so don’t delay receiving it.

Outlook / Prognosis

Why is influenza (flu) more dangerous for elderly or chronically ill people?

People over 65 and those with chronic(long-term) illnesses have a hard time fighting influenza because the body’s system for fighting infections is often weakened by age and illness. In older people, influenza is also more likely to lead to:

  • Pneumonia.
  • Hospitalization.
  • Death.

Living With

When should you contact your healthcare provider if you have influenza (flu)?

Because of the importance of taking flu medicines within 48 hours of coming down with symptoms, call immediately if you think you have the flu. If you continue to feel unwell after you have been treated for the flu, you should call your doctor’s office. If you find yourself feeling better, and then getting sick again, you should also contact your doctor. The flu might have left you with some kind of secondary illness, like a sinus infection.

When should you go back to work or school if you have had the flu?

You are contagious for one day before starting feeling ill, and for 5 to 7 days while you have the flu symptoms. At the very least, you should stay at home until you are able to go 24 hours without taking something for fever. If you have other severe symptoms, such as incessant (non-stop) cough or shortness of breath, you should stay at home.

Last reviewed by a Cleveland Clinic medical professional on 11/24/2019.


  • Centers for Disease Control and Prevention. Seasonal Influenza (Flu). ( Accessed 11/25/2019.
  • National Institute of Allergy & Infectious Disease. Flu (Influenza). ( Accessed 11/25/2019.
  • National Institute of Aging. All about the flu and how to prevent it. ( Accessed 11/25/2019.
  • National Foundation for Infectious Diseases. Flu (Influenza). ( Accessed 11/25/2019.
  • Centers for Disease Control and Prevention. What you should know about flu antiviral drugs. ( Accessed 11/25/2019.
  • U.S. Food & Drug Administration. [email protected]: FDA Approved Drug Products. ( Accessed 11/25/2019.
  • Centers for Disease Control and Prevention. Influenza Antiviral Medications: Summary for Clinicians. ( Accessed 11/25/2019.

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