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Influenza (Seasonal Flu)

Topic Overview

What is influenza (flu)?

Influenza (flu) is a viral infection. People often use the term "flu" to describe any kind of mild illness, such as a cold or a stomach virus, that has symptoms like the flu. But the real flu is different. Flu symptoms are usually worse than a cold and last longer. The flu usually does not cause vomiting or diarrhea in adults.

Most flu outbreaks happen in late fall and winter. Because symptoms may not start for a couple of days, you may pass the flu to someone before you know you have it.

What causes the flu?

The flu is usually caused by influenza viruses A and B. There are different strains of the flu virus every year.

What are the symptoms?

The flu causes a fever, body aches, a headache, a dry cough, and a sore or dry throat. You will probably feel tired and less hungry than usual. The symptoms usually are the worst for the first 3 or 4 days. But it can take 1 to 2 weeks to get completely better.

It usually takes 1 to 4 days to get symptoms of the flu after you have been around someone who has the virus.

Most people get better without problems. But sometimes the flu can lead to a bacterial infection, such as an ear infection, a sinus infection, or bronchitis. Less often, the flu may cause a more serious problem, such as pneumonia.

Certain people are at higher risk of problems from the flu. They include young children, pregnant women, older adults, and people with long-term illnesses or with impaired immune systems that make it hard to fight infection.

How is the flu diagnosed?

Your doctor will ask you about your symptoms and examine you. This usually gives the doctor enough information to find out if you have the flu, especially if many cases of a similar illness have occurred in the area and the local health department reports a flu outbreak.

In some cases, the doctor may do a blood test or take a sample of fluid from your nose or throat to find out what type of flu virus you have.

How is it treated?

Most people can treat flu symptoms at home. Home treatment includes resting, drinking plenty of fluids, and taking medicine to lower your fever. But some people need to go to the hospital for treatment. They may have severe symptoms or get pneumonia. Or the flu infection may make an existing health problem worse.

If you think you have the flu, your doctor may be able to give you medicine that can make the symptoms milder. It's best to start taking it within 2 days of your first symptoms.

Can the flu be prevented?

You can help prevent the flu by getting the flu vaccine every year. It's best to get the vaccine as soon as it's available.

The U.S. Centers for Disease Control and Prevention recommends that everyone 6 months old and older should get a flu vaccine. The vaccine is especially important for people who are at higher risk of problems from the flu, including:

  • Young children.
  • Adults ages 50 and older.
  • Adults and children who have long-term health problems or an impaired immune system.
  • Women who will be pregnant during the flu season.

The flu vaccine is also important for health care workers and anyone who lives or works with a person who is at higher risk of problems from the flu.

The vaccine usually prevents most cases of the flu. But even if you do get the flu after you've had the vaccine, your symptoms will be milder and you'll have less chance of problems from the flu. You cannot get the flu from the flu vaccine.

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Cause

The flu is caused by the influenza virus. Common classes of the influenza virus are type A and type B, each of which includes several subtypes or strains. Type A is usually responsible for the annual outbreaks that typically occur in the late fall and early winter.

The influenza virus changes often, so having flu caused by one strain does not give you full immunity to other strains.

  • Widespread outbreaks of the flu usually follow significant changes (called antigenic shifts) in the virus and occur about every 10 years. People who get the flu tend to become much sicker when a shift in the flu virus occurs.
  • Minor changes in the virus (called antigenic drifts) occur nearly every year.

The virus is spread from person to person through:

  • Direct contact, such as shaking hands.
  • Small droplets that form when a person sneezes or coughs.
  • Contact with objects such as handkerchiefs that have been in contact with fluids from an infected person's nose or throat.

When are you contagious?

If you are infected with the flu, you are most likely to pass it to someone else from 1 day before symptoms start and up to 7 days after symptoms develop. Children may be infectious for longer than 7 days after symptoms start.

Symptoms usually develop 1 to 4 days after you are infected. Because symptoms may not develop for a couple of days, you may pass the flu to someone before you know you have it.

Symptoms

The symptoms of influenza (flu) appear suddenly and often include:

  • Fever of 100.4°F (38°C) to 104°F (40°C), which can reach 106°F (41°C) when symptoms first develop. Fever is usually continuous, but it may come and go. Fever may be lower in older adults than in children and younger adults. When fever is high, other symptoms usually are more severe.
  • Body aches and muscle pain (often severe), commonly in the back, arms, or legs.
  • Headache.
  • Pain when you move your eyes.
  • Fatigue, a general feeling of sickness (malaise), and loss of appetite.
  • A dry cough, runny nose, and dry or sore throat. You may not notice these during the first few days of the illness when other symptoms are more severe. As your fever goes away, these symptoms may become more evident.

Some people get infected with the flu virus but do not have any symptoms.

Influenza usually does not cause symptoms in the stomach or intestines, such as vomiting and diarrhea.

Other conditions have symptoms similar to the flu, such as the common cold, bacterial infections, and infectious mononucleosis.

What Happens

Influenza (flu) usually comes on suddenly. In many cases people can pinpoint the hour when symptoms started. Symptoms develop 1 to 4 days after you are infected, and they include:

  • Fever, which lasts for about 3 days. Fever is usually slightly lower on the 2nd and 3rd days but may last up to 8 days.
  • Cough, runny nose, and sore throat, which become more noticeable as fever and other symptoms decrease. These symptoms usually last 3 to 4 days after the fever goes down. A dry, hacking cough may linger for up to 10 days after other symptoms are gone.

Complete recovery may take 1 to 2 weeks or longer. Fatigue and weakness can last for several weeks.

Complications of influenza may develop in anyone, but they are much more likely in older adults and people who have other health problems, especially heart and lung diseases.

What Increases Your Risk

Anyone exposed to an influenza (flu) virus can become infected. These viruses are contagious and spread easily among people in groups, such as in nursing homes, hospitals, shelters, schools, and day cares. Working, visiting, or living in any of these areas increases your risk of getting the flu.

The risk of having severe symptoms and complications is higher for:

  • Children younger than 2 years of age.
  • Adults age 65 and older.
  • Pregnant women.
  • People who have chronic obstructive pulmonary disease (COPD), other lung diseases, or heart failure.
  • People who have a medical condition (such as acquired immunodeficiency syndrome [AIDS]) or who are using a medicine that impairs the immune system.

When should you call your doctor?

Call 911 or other emergency services if:

  • You are having trouble breathing, or you feel very short of breath.
  • You have a severe headache or stiff neck and are confused or having trouble staying awake.

Call your doctor if:

  • You have an extremely high fever.
  • Your fever lasts longer than 3 days.
  • Your child is 3 months of age or younger and has a fever of 100.4°F (38°C) or higher.
  • Your cough lasts more than 7 to 10 days after other symptoms are gone.
  • You are coughing up yellow, green, rust-colored, or bloody mucus.
  • You are finding it harder and harder to breathe.
  • Wheezing develops.
  • New pain develops or pain narrows to one area, such as an ear, the throat, the chest, or the sinuses.
  • Symptoms don't go away, even with home treatment.
  • Symptoms become more severe or frequent.

Watchful waiting

In most healthy people, the flu will go away in 5 to 7 days, although fatigue can last much longer. You may feel very sick, but home treatment is usually all that is needed. If it's flu season, you may just want to treat your symptoms at home. Watch closely for symptoms of a bacterial infection, such as nasal drainage that changes from clear to colored after 5 to 7 days and symptoms that return or get worse.

Early treatment with an antiviral medicine may reduce the severity of influenza and may prevent serious flu-related complications. It's best to start these medicines right away. Babies, older adults, and people who have chronic health problems are more likely to have complications from the flu, and they may need to see a doctor for care beyond home treatment. But not all antiviral medicines work against all strains of the flu. Talk to your doctor if you think you may need an antiviral medicine.

Call your doctor if you think your symptoms are caused by something other than the flu.

Exams and Tests

Doctors can diagnose influenza (flu) using your symptoms alone if many cases of a similar illness have occurred in the community and if the local health department has confirmed a flu outbreak.

Tests to confirm you have the flu and to find out the type of virus may be important if:

  • The results may affect treatment decisions. During a confirmed flu outbreak, though, testing may not be needed even if treatment with an antiviral medicine is being considered.
  • In addition to flu-like symptoms, you have any unusual symptoms that suggest another condition.
  • Health authorities have not identified any other cases of flu in your area.

A rapid flu test gives results quickly. Although this test is not always correct, it can be useful when deciding whether to use an antiviral medicine.

Treatment Overview

In most healthy people, influenza (flu) will go away in 5 to 7 days. The worst symptoms usually last 3 to 4 days. Home treatment to ease symptoms and prevent complications is usually all that is needed.

But some people need treatment in the hospital. They may have severe symptoms or get pneumonia. Or the flu infection may make an existing health problem worse.

Antiviral medicines may help:

  • Reduce the severity and duration of symptoms caused by infection with influenza A or B virus.
  • Shorten the length of the illness.
  • Control outbreaks of the flu in nursing homes.
  • Reduce the spread of the virus to people at high risk for severe complications of the flu (high-risk groups).
  • Reduce complications from the flu.

People at high risk of complications are encouraged to contact a doctor within 48 hours of their first symptoms to find out whether they need medicine to shorten the illness. They also should call a doctor to receive medicine if they have been exposed to the flu.

Prevention

You can help prevent influenza by getting immunized with an influenza vaccine each year as soon as it's available.

There are different ways to get a flu vaccine. Ask your doctor what is right for you.

Even if a flu vaccine does not prevent the flu, it can reduce the severity of flu symptoms and decrease the risk of complications. Studies have found that the flu vaccine results in fewer days missed from work and fewer visits to a doctor for respiratory infections, and it reduces the number of people who develop complications from the flu, such as pneumonia.footnote 1 And the flu vaccine can help protect the babies of women who got the vaccine while they were pregnant.footnote 2, footnote 3

In spite of these results, many people choose not to get a flu vaccine. Some do not get the vaccine because of myths they believe about the flu or the vaccines. These include beliefs that the flu is a minor illness or that the vaccine causes the flu. The vaccine may cause side effects, such as soreness or fever, but they are usually minor and do not last long.

Although antiviral medicines sometimes prevent the flu, they do not work in the same way as a yearly immunization and should not replace a flu vaccine.

The person who gives the vaccine may tell your child or you not to get it if your child or you:

  • Have a severe allergy to any part of the vaccine.
  • Have had a serious reaction to a previous dose of flu vaccine.
  • Have had Guillain-Barré syndrome.
  • Are sick. If you are ill and have a fever, wait until you're better before you get a flu vaccine.

People with egg allergies can get a flu vaccine that is recommended for their age. If their egg allergy is mild, they can get the vaccine anywhere it is offered. But people with severe allergies to eggs should be vaccinated at a clinic with health care providers trained to manage severe allergic reactions.

Almost every community has a program that offers flu vaccines at low cost during the flu season. You also can get a flu vaccine during a routine visit to a doctor or pharmacy. Many health clinics have set hours at the start of the flu season for people to get flu vaccines without needing to make an appointment.

Other ways to reduce your risk for the flu or flu complications

Increase your chance of staying healthy by:

  • Washing your hands often, especially during winter months when the flu is most common.
  • Keeping your hands away from your nose, eyes, and mouth. Viruses are most likely to enter your body through these areas.
  • Eating a healthy and balanced diet.
  • Getting regular exercise.
  • Not smoking. Smoking irritates the lining of your nose, sinuses, and lungs, which may make you susceptible to complications of the flu.footnote 4

Using antiviral medicines to prevent the flu

Two antiviral medicines (oseltamivir and zanamivir) can help prevent the flu caused by influenza A and B viruses. These medicines may also reduce the length of the illness if they are given as soon as possible after the first symptoms. During a flu outbreak, these medicines may be given at the same time as a flu vaccine and for 2 weeks after while your body produces antibodies to protect you from the virus.footnote 5 The influenza medicines are usually given to people who are very sick with the flu or to those who are likely to have complications from the flu. But they may also be used for a person who has been sick with the flu for less than 48 hours. These medicines are taken by mouth (pill) or inhaled into the lungs (inhaler).

The antiviral medicines amantadine and rimantadine have been used to prevent flu caused by influenza A. But for the past few years the U.S. Centers for Disease Control and Prevention (CDC) has advised doctors not to use these medicines to treat or prevent the flu. These medicines have not worked against most types of the flu virus. Amantadine and rimantadine do not protect against influenza B. Be sure to talk with your doctor about the medicine that is best for you.

Home Treatment

If you have influenza, you can expect the illness to go away on its own in about 7 to 10 days. In the meantime, you can take steps to feel better:

  • Get extra rest. Extra rest can help you feel better. It will also help you avoid spreading the virus to others.
  • Drink plenty of fluids to replace those lost from fever. Fluids also ease a scratchy throat and keep nasal mucus thin. Water, soup, fruit juice, and hot tea with lemon are all good choices.
  • To help clear a stuffy nose, breathe moist air from a hot shower or from a sink filled with hot water.
  • If the skin around your nose and lips becomes sore from repeated rubbing with tissues, apply a bit of petroleum jelly to the area. Using disposable tissues that contain lotion also may help.
  • Elevate your head at night with an extra pillow if coughing keeps you awake.
  • Avoid smoking and breathing secondhand smoke. This is good advice anytime, but it is especially important when you have a respiratory infection like a cold or the flu.
  • Try an over-the-counter medicine to help relieve your symptoms. Be safe with medicines. Read and follow all instructions on the label. Cough and cold medicines may not be safe for young children or for people who have certain health problems.
    • To relieve body aches and headache or to lower fever, try acetaminophen or ibuprofen. If you give medicine to your baby, follow your doctor's advice about what amount to give.
    • For a stuffy nose, try a nasal or oral decongestant (such as Afrin or Sudafed PE) or a steroid nasal spray (such as Nasacort). Do not use a nasal decongestant longer than the label says. Overuse can cause a "rebound effect". It makes your mucous membranes swell up more than before you used the spray.
    • If you have a dry, hacking cough, you can try a cough suppressant (such as Delsym, Mucinex DM, or Robitussin) to help stop the cough reflex.
    • To soothe a sore throat, use throat lozenges or plain, hard candy.
    • Avoid antihistamines. They do not treat flu symptoms and may make nasal drainage thicker.
  • If fever is uncomfortable, sponge your body with lukewarm water to reduce fever. Do not use cold water or ice. Lowering the fever will not make your symptoms go away faster, but it may make you more comfortable.
  • Protect others.
    • Wash your hands frequently.
    • Wear a mask around other people.
    • Stay away from public places to limit spreading the flu to other people.

Call your doctor if:

  • Your symptoms improve but then seem to get worse again.
  • You have symptoms of a bacterial infection, such as feeling short of breath; a new or worse cough that produces yellow, green, rust-colored, or bloody mucus; persistent fever; ear pain, sore throat, or sinus pain.

Medications

Some antiviral medicines reduce the severity and shorten the duration of influenza (flu) symptoms by 1 to 1½ days if given within 48 hours of the first symptoms.footnote 5 These medicines are not intended to substitute for getting a flu vaccine each year. Rather, antiviral medicines may help control outbreaks and prevent the spread of infection, especially in people at high risk for flu complications.

The antiviral medicines oseltamivir and zanamivir are used to prevent and treat influenza A and B infections. They can reduce the severity and shorten the duration of flu symptoms. Amantadine and rimantadine have been used to help prevent and treat the flu caused by influenza A (but not influenza B) infection. But for the past few years the U.S. Centers for Disease Control and Prevention (CDC) has advised doctors not to use amantadine or rimantadine to treat or prevent the flu.footnote 4 These medicines have not worked against most types of the flu virus. It is important to talk with your doctor about the medicine that is best for you.

Medicine choices

Two antiviral medicines, oseltamivir and zanamivir, can treat and prevent influenza A and B infections.

What to think about

The effectiveness of antiviral medicines can vary from year to year. Some years a medicine may not work against the types of influenza virus causing symptoms. Your doctor can help you decide whether antiviral medicines are likely to help you.

Most people do not need antiviral medicines. They recover from influenza without having complications.

But since most people who have the flu feel quite sick, some people may choose to take medicine even if they are at low risk for complications.

You cannot prevent the flu or make yourself feel better faster by taking:

  • Antibiotics. For more information, see the topic Using Antibiotics Wisely.
  • Large doses of vitamins and minerals, such as vitamin C or zinc.
  • Herbal remedies, such as echinacea.

References

Citations

  1. Nichol KL, et al. (2007). Effectiveness of influenza vaccine in the community-dwelling elderly. New England Journal of Medicine, 357(14): 1373–1381.
  2. Eick AA, et al. (2011). Maternal influenza vaccination and effect on influenza virus infection in young infants. Archives of Pediatrics and Adolescent Medicine, 165(2): 104–111.
  3. Zhau D, et al. (2014). Effectiveness of seasonal influenza vaccination during pregnancy in preventing influenza infection in infants, England, 2013/14. Euro Surveillance, 19(45): 20959. http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20959. Accessed February 10, 2015.
  4. Centers for Disease Control and Prevention (2014). Influenza Antiviral Medications: Summary for Clinicians. http://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm. Accessed April 11, 2014.
  5. Jones JT, et al. (2014). Neuraminidase inhibitors for preventing and treating influenza in adults and children. Cochrane Database of Systematic Review (4). http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD008965.pub4/pdf/standard. Accessed February 10, 2015.

Credits

Current as of: March 9, 2022

Author: Healthwise Staff
Medical Review:
E. Gregory Thompson MD - Internal Medicine
John Pope MD - Pediatrics
Adam Husney MD - Family Medicine
Kathleen Romito MD - Family Medicine
Christine Hahn MD - Epidemiology

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