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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.

What causes the flu?

The flu is 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. In rare cases, 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.

If you think you have the flu, your doctor may be able to give you medicine that can make the symptoms milder. But you need 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. You can get the vaccine as a shot or in a spray that you breathe in through your nose.

The 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:1

  • Children 6 months through 4 years of age.
  • 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 recommended for health care workers and anyone who lives or works with a person who is at higher risk of problems from the flu. Your doctor can help you decide if the flu vaccine is a good choice for you.

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.

Frequently Asked Questions

Learning about influenza (flu):

Being diagnosed:

Getting treatment:

Ongoing concerns:

Taking care of yourself:

Health Tools Health Tools help you make wise health decisions or take action to improve your health.

Health Tools help you make wise health decisions or take action to improve your health.
Decision Points focus on key medical care decisions that are important to many health problems. Decision Points focus on key medical care decisions that are important to many health problems.
  Flu Vaccines: Should I Get a Flu Vaccine?
  Flu: Should I Take Antiviral Medicine?

Cause

The flu is caused by the influenza virus. Doctors classify the virus as influenza 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°F (37.8°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.

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 To Call a Doctor

Call 911 or other emergency services if:

  • A baby younger than 3 months has a high fever.
  • You are having trouble breathing or 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 for longer than 3 days.
  • You are finding it harder and harder to breathe.
  • Wheezing develops.
  • New pain develops or pain localizes to one area, such as an ear, the throat, the chest, or the sinuses.
  • Symptoms persist in spite of 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. Although you may feel very sick, home treatment is usually all that is needed. If it is 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 (within 48 hours of your first symptoms) with antiviral medicines may reduce the severity of influenza and may prevent serious flu-related complications.2 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.

Who to see

These health professionals can diagnose and treat the flu:

A doctor who specializes in treating infectious diseases may be needed if the diagnosis is not clear or if severe complications develop.

To prepare for your appointment, see the topic Making the Most of Your Appointment.

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 determine the type of virus may be important if:

  • The results may affect treatment decisions. During a confirmed flu outbreak, however, 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.

Testing may involve blood tests (rarely used) or a culture to identify the virus. Some cultures take 24 to 48 hours for results, so they will not help your doctor decide whether to prescribe an antiviral medicine. A rapid flu test is now available that gives results in 30 minutes. Although this test is not 100% accurate, 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.

Antiviral medicines can be taken to:

  • 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.

Click here to view a Decision Point.Flu: Should I Take Antiviral Medicine?

If medicines are not used, contact your doctor if symptoms of a complication develop.

What to think about

Yearly immunization with the inactivated influenza vaccine(What is a PDF document?) (flu shot) or the nasal spray flu vaccine(What is a PDF document?) prevents flu infection and its complications in most people. The nasal spray vaccine is approved for use by healthy people who are 2 to 49 years old. You should not get the nasal spray vaccine if you:

  • Have heart disease.
  • Have lung disease, including asthma.
  • Have diabetes or kidney disease.
  • Have a disease or take a medicine that causes problems with your immune system.
  • Have a condition (such as a seizure disorder or cerebral palsy) that can cause breathing or swallowing problems.
  • Are pregnant.
  • Are younger than age 20 and you take aspirin or products with aspirin in them.

Before getting a flu vaccine, talk to your doctor if:

  • You ever had a serious allergic reaction to eggs or to a previous dose of influenza vaccine.
  • You have had Guillain-Barré syndrome.

The yearly immunization rate is typically low for people younger than 65 who are at high risk of developing the flu. All people in high-risk groups and those who could transmit the virus to them because of regular contact are strongly encouraged to get the flu vaccine.

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.

Click here to view a Decision Point.Flu Vaccines: Should I Get a Flu Vaccine?

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 shot or dose of the nasal spray vaccine.

Prevention

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

Most healthy people ages 2 through 49 years can choose to get the nasal spray form(What is a PDF document?) of the vaccine (such as FluMist) instead of the flu shot. The nasal spray vaccine contains components of live viruses, so it should not be given to people with certain long-term (chronic) health conditions, such as heart or lung problems. Close contacts of these people in high-risk categories can be given either type of vaccine, with one rare exception. Immunization with the inactivated virus (flu shot) is preferred over the nasal spray vaccine for close contacts of people with severely impaired immune systems during times when a protected environment is needed. This avoids the risk of transmitting an active flu virus from the nasal spray vaccine. If the nasal spray vaccine is used, contact with anyone in this high-risk group should be avoided for 7 days. For close contacts of people in all other high-risk categories, vaccination with either the flu shot or the nasal spray is considered safe.

You should not get the nasal spray if you:

  • Have heart disease.
  • Have lung disease, including asthma.
  • Have diabetes or kidney disease.
  • Have a disease or take a medicine that causes problems with your immune system.
  • Have a condition (such as a seizure disorder or cerebral palsy) that can cause breathing or swallowing problems.
  • Are pregnant.
  • Are younger than age 20 and you take aspirin or products with aspirin in them.

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 shot 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.3 And the flu vaccine can help protect the babies of women who got the vaccine while they were pregnant.4

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 shot may cause side effects , such as soreness or fever, but they are usually minor and do not last long.

Before getting a flu vaccine, talk to your doctor if:

  • You ever had a serious allergic reaction to eggs or to a previous dose of influenza vaccine.
  • You have had Guillain-Barré syndrome.
  • Your child has ever had a seizure.

Because the nasal spray vaccine is more expensive than a flu shot, it may not be covered by your health insurance plan. Check with your insurance company.

Click here to view a Decision Point.Flu Vaccines: Should I Get a Flu Vaccine?

Other ways to reduce your risk of getting the flu or developing 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.
  • Taking probiotics. One study has shown that taking probiotics helps prevent influenza symptoms and reduce antibiotic use in children.5

Using antiviral medicines to prevent the flu

Two antiviral medicines (zanamivir and oseltamivir) 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 no more than 48 hours 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. 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.6 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.

Click here to view a Decision Point.Flu: Should I Take Antiviral Medicine?

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. Bed 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. Hot tea with lemon, water, fruit juice, and soup are all good choices.
    • 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. You may also take acetaminophen or ibuprofen to lower fever. Follow all instructions on the label. If you give medicine to your baby, follow your doctor’s advice about what amount to give. Do not give aspirin to anyone younger than 20 because of the risk of Reye syndrome.
  • To relieve body aches and headache, take acetaminophen or ibuprofen.
    Over-the-Counter Medicine Precautions
    Quick Tips: Giving Over-the-Counter Medicines to Children
  • To help clear a stuffy nose, breathe moist air from a hot shower or from a sink filled with hot water.
  • Try a decongestant or nasal spray if your main symptom is a stuffy nose. Look for a single-ingredient decongestant that contains phenylephrine. If nasal drainage is thick, a decongestant that contains guaifenesin may help keep it thin and draining. Before you use these medicines, check the label. They may not be safe for young children or for people who have certain health problems. Do not use medicated nasal sprays or drops more often than directed, and don't use them for longer than 3 days.
  • Avoid antihistamines. They do not treat flu symptoms and may make nasal drainage thicker.
  • 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.
  • Use cough drops or plain, hard candy to help ease coughing.
  • If you have a dry, hacking cough, you can try an over-the counter cough medicine with dextromethorphan. Be careful with cough and cold medicines. They may not be safe for young children or for people who have certain health problems, so check the label first. If you do use these medicines, always follow the directions about how much to use based on age and weight.
  • 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.

Call your doctor if:

  • Your symptoms improve but then seem to get worse again.
  • You develop symptoms of a bacterial infection, such as a new or worse cough that produces yellow, green, rust-colored, or bloody mucus; persistent fever, ear pain, sore throat, sinus pain, or productive cough; or nasal drainage that changes from clear to colored after 7 to 10 days.

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.2 These medicines are not intended to substitute for getting a flu immunization each year. Rather, antiviral medicines are important for controlling outbreaks and preventing the spread of infection, especially in people at high risk for developing flu complications.

The antiviral medicines zanamivir and oseltamivir are used to prevent and treat influenza A and B infections. They can reduce the severity and shorten the duration of flu symptoms.7 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.6 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.

Click here to view a Decision Point.Flu: Should I Take Antiviral Medicine?

Medication choices

Two types of antiviral medicines can treat influenza 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 developing complications.

But given how sick most people are when they have the flu, 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.

Other Places To Get Help

Organizations

American Lung Association
1301 Pennsylvania Avenue NW
Suite 800
Washington, DC  20004
Phone: 1-800-LUNG-USA (1-800-586-4872)
1-800-548-8252 (to speak with a lung professional)
(212) 315-8700
Email: info@lungusa.org
Web Address: www.lungusa.org
 

The American Lung Association provides programs of education, community service, and advocacy. Some of the topics available include asthma, tobacco control, emphysema, infectious disease, asbestos, carbon monoxide, radon, and ozone.


Centers for Disease Control and Prevention (CDC)
1600 Clifton Road
Atlanta, GA  30333
Phone: 1-800-CDC-INFO (1-800-232-4636)
TDD: 1-888-232-6348
Email: cdcinfo@cdc.gov
Web Address: www.cdc.gov
 

The Centers for Disease Control and Prevention (CDC) is an agency of the U.S. Department of Health and Human Services. The CDC works with state and local health officials and the public to achieve better health for all people. The CDC creates the expertise, information, and tools that people and communities need to protect their health—by promoting health, preventing disease, injury, and disability, and being prepared for new health threats.


KidsHealth for Parents, Children, and Teens
10140 Centurion Parkway North
Jacksonville, FL  32256
Phone: (904) 697-4100
Fax: (904) 697-4220
Web Address: www.kidshealth.org
 

This website is sponsored by the Nemours Foundation. It has a wide range of information about children's health, from allergies and diseases to normal growth and development (birth to adolescence). This website offers separate areas for kids, teens, and parents, each providing age-appropriate information that the child or parent can understand. You can sign up to get weekly emails about your area of interest.

National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health
NIAID Office of Communications and Government Relations
6610 Rockledge Drive, MSC 6612
Bethesda, MD  20892-6612
Phone: 1-866-284-4107 toll-free
Phone: (301) 496-5717
Fax: (301) 402-3573
TDD: 1-800-877-8339
Web Address: www.niaid.nih.gov
 

The National Institute of Allergy and Infectious Diseases conducts research and provides consumer information on infectious and immune-system-related diseases.

National Institute on Aging
Building 31, Room 5C27
31 Center Drive, MSC 2292
Bethesda, MD  20892
Phone: 1-800-222-2225
Phone: (301) 496-1752
TDD: 1-800-222-4225 (TTY)
Email: niaic@nia.nih.gov
Web Address: www.nia.nih.gov
 

The National Institute on Aging (NIA), one of the centers of the U.S. National Institutes of Health, leads a broad scientific effort to understand the nature of aging and to extend the healthy, active years of life. The NIA funds research and provides information about health and research advances to the public and interested groups.

U.S. Department of Health and Human Services
200 Independence Avenue SW
Washington, DC 20201
Phone: 1-800-CDC-INFO (1-800-232-4636)
TDD: 1-888-232-6348
Email: cdcinfo@cdc.gov
Web Address: www.flu.gov
 

Flu.gov is a website sponsored by the U.S. Department of Health and Human Services. The site provides information about the flu, including seasonal flu, H1N1 (swine) flu, and avian (bird) flu. It offers answers to frequently asked questions, and it has special sections for older adults, caregivers, travelers, pregnant women, and health professionals. The website also offers guidance on vaccinations.

References

Citations

  1. Centers for Disease Control and Prevention (2010). Prevention and control of influenza with vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR, 59(RR-08): 1–62. Also available online: http://www.cdc.gov/mmwr/pdf/rr/rr5908.pdf. [Errata in MMWR, 59(31): 993 and MMWR, 59(35): 1147.]
  2. Jefferson T (2009). Influenza, search date June 2008. Online version of BMJ Clinical Evidence: http://www.clinicalevidence.com.
  3. Nichol KL, et al. (2007). Effectiveness of influenza vaccine in the community-dwelling elderly. New England Journal of Medicine, 357(14): 1373–1381.
  4. Zaman K, et al. (2008). Effectiveness of maternal influenza immunization in mothers and infants. New England Journal of Medicine, 359(15): 1555–1564.
  5. Leyer GJ, et al. (2009). Probiotic effects on cold and influenza-like symptom incidence and duration in children. Pediatrics, 124(2): e172–e179.
  6. Centers for Disease Control and Prevention (2006). CDC Health Alert: CDC Recommends Against the Use of Amantadine and Rimantadine for the Treatment or Prophylaxis of Influenza in the United States During the 2005–06 Influenza Season. Available online: http://www.cdc.gov/flu/han011406.htm.
  7. Cooper NJ, et al. (2003). Effectiveness of neuraminidase inhibitors in treatment and prevention of influenza A and B: Systematic review and meta-analyses of randomised controlled trials. BMJ, 326(7401): 1235.

Other Works Consulted

  • U.S. Centers for Disease Control and Prevention (2011). Maternal and infant outcomes among severely ill pregnant and postpartum women with 2009 pandemic influenza A (H1N1)—United States, April 2009–August 2010. Morbidity and Mortality Weekly Report, 60(35): 1193–1196.

Credits

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Christine Hahn, MD - Epidemiology
Last Revised October 17, 2011

Last Revised: October 17, 2011

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Christine Hahn, MD - Epidemiology

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