Influenza

What is influenza (flu)?

Influenza (flu) is a viral infection of the nose, throat, and lungs. It comes on suddenly and causes symptoms such as fever, body aches, headache, fatigue, loss of appetite, dry cough, and sore or dry throat. The flu is not the same as the common cold; flu symptoms are usually more severe, and you will often miss more work or school than you would with a cold.

Most people recover without problems, but sometimes the illness leads to a bacterial infection, such as an ear infection, sinus infection, or bronchitis. Good home treatment may help prevent these infections. More serious complications, most commonly pneumonia, may develop in some people. People at increased risk for complications that require hospitalization are young children, adults age 65 and older, and those with serious medical problems.

What causes the flu?

The flu is caused by influenza viruses A and B. Type A is usually responsible for the outbreaks that occur annually.

These viruses are constantly changing, producing what are called subtypes or strains that are different from the original virus but retain some of its characteristics. Strains of influenza virus that cause the flu may differ from year to year.

What are the symptoms?

The flu causes a fever, cough, body aches, headache, and fatigue. These symptoms usually last for 3 to 4 days, after which you may have a dry cough, runny nose, and sore or scratchy throat for another week or so. The incubation period—the time from exposure to the flu virus until you develop symptoms—is 1 to 4 days.

Although people often use the term "flu" to describe any kind of mild illness that has flu-like symptoms (such as the common cold or a stomach virus), the flu is a distinct viral illness with specific symptoms, and it does not usually cause vomiting and diarrhea. It tends to occur at a particular time of year—late fall and winter.

How is the flu diagnosed?

Doctors usually can diagnose influenza using your symptoms alone, especially if many cases of a similar illness have occurred in the community and the local health department has confirmed a flu outbreak. Routine testing of people who have typical flu symptoms is becoming more available and more useful.

How is it treated?

Home treatment is usually all that is needed for flu symptoms. However, antiviral medications are available to reduce the duration and severity of symptoms. These medications are especially useful for older adults and others who are at risk for developing complications from the flu. These medications need to be started within 2 days of your first symptoms. Not all antiviral medicines work against all strains of the flu.¹

Can the flu be prevented?

You can help prevent the flu by getting immunized with an influenza vaccine each year, ideally in October or November. The standard vaccine, known as the "flu shot," is given by injection. It can be given after age 6 months to anyone who wishes to help prevent the flu. The flu shot is recommended for:²

• All children age 6 months to 59 months.
• All adults age 50 and older.
• Adults and children who have health conditions such as asthma, chronic heart or lung disorders, or an impaired immune system.
• Women who will be pregnant during the flu season.
• Close contacts (including household members and health care workers) of anyone in a high-risk category, which includes all children 59 months of age and younger.

FluMist, a live-virus vaccine in the form of a nasal spray, is an alternative to the flu shot that is now available for healthy children and adults between the ages of 5 and 49, except pregnant women. FluMist should not be given to close contacts of people with severely impaired immune systems (such as those who have had a recent bone marrow transplant) to avoid transmitting the virus after being vaccinated.

Frequently Asked Questions

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. These strains are different from the original virus but retain some of its characteristics. 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?

People who have the flu are most likely to pass it to someone else from 1 day before to 5 days after symptoms develop. Children may be infectious for up to 6 days before symptoms develop.

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.

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

• Fever of 100° (37.8°) to 104° (40°), which can reach 106° (41°) 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.

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.

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 developing severe symptoms and complications is higher for:

• Children younger than 4 years of age.
• Adults age 65 and older. About 90% of the deaths caused by flu occur in this age group.³
• Women who will be in their second or third trimester of pregnancy during the flu season.³
• 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 medication that impairs the immune system.

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 medications may reduce the severity of influenza. However, it is not certain whether antiviral medications can prevent serious flu-related complications.⁴ 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. Also, not all antiviral medicines work against all strains of the flu. Talk to your doctor if you think you may need an antiviral medicine.¹

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

Who To See

These doctors can diagnose and treat the flu:

• Family physician
• Internist
• Pediatrician
• Nurse practitioner
• Physician assistant

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.

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 medication 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 medication. 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 medication.

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 medications 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 medication to shorten the illness. They also should call a doctor to receive medication if they have been exposed to the flu.

If medications 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?) (FluMist) prevents flu infection and its complications in most people. FluMist is approved for use by healthy people who are 5 to 49 years old. You should not use FluMist 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 had Guillain-Barré syndrome.
• Are pregnant.
• Have ever had an allergic reaction to FluMist or to eggs.
• Are younger than age 20 and you take aspirin or products with aspirin in them.

The yearly vaccination 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 shot.²

Almost every community has a program that offers flu shots at low cost during the months of October and November. You also can get a flu shot during a routine visit to a doctor during the same months. Many health clinics have set hours during October and November for people to get flu shots without needing to make an appointment.

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 FluMist.

You can help prevent influenza by getting immunized with an influenza vaccine each year, ideally in October or November. The inactivated influenza vaccine (What is a PDF document?) commonly known as the "flu shot," is given by injection. This form of the vaccine effectively prevents most cases of the flu, although success rates vary according to age, health status, and how closely the virus strains contained in the vaccine match those that are circulating through the population.

Even if a flu shot does not prevent the flu, the vaccine 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.²

In spite of these results, many people choose not to get a flu shot. Some do not get the shot because of myths they believe about the flu or the vaccine. The flu shot may cause side effects in some people, but they are usually minor and do not last long.

An alternative to the flu shot is now available. In 2003, the U.S. Food and Drug Administration (FDA) approved a nasal spray flu vaccine (What is a PDF document?). This vaccine, called FluMist, is available to healthy children and adults between the ages of 5 and 49 years. 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.²

You should not use FluMist 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 had Guillain-Barré syndrome.
• Are pregnant.
• Have ever had an allergic reaction to FluMist or to eggs.
• Are younger than age 20 and you take aspirin or products with aspirin in them.

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

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.

Using antiviral medications to prevent the flu

The antiviral medications amantadine and rimantadine can prevent flu caused by influenza A. However, amantadine and rimantadine do not work against all flu strains.¹ It is important to talk with your doctor about the medication that is best for you.

Two other antiviral medications (zanamivir and oseltamivir) can prevent the flu caused by influenza A and B viruses. These medications 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 medications may be given at the same time as the flu shot and for 2 weeks after while your body produces antibodies to protect you from the virus. These medications are taken by mouth (pill) or inhaled into the lungs (inhaler).

Some people cannot have a flu shot because they are allergic to eggs, from which the vaccine is made.

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, take acetaminophen or ibuprofen to lower it. You may also 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.
• To relieve body aches and headache, take acetaminophen or ibuprofen.
• 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. Do not use medicated nasal sprays or drops more often than directed, and don't use them for longer than 3 days.
• Breathe moist air from a hot shower or from a sink filled with hot water to help clear a stuffy nose.
• 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.
• Take a nonprescription cough medicine that contains dextromethorphan if you develop a dry, hacking cough. Some products contain a high percentage of alcohol. Use them with caution.
• Elevate your head at night with an extra pillow if coughing keeps you awake.
• Avoid smoking and breathing secondhand smoke. This is good advice any time, 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.

Antiviral medications reduce the severity and shorten the duration of influenza (flu) symptoms by 1 or 1½ days if given within 48 hours of the first symptoms.² These medications are not intended to substitute for getting a flu immunization each year. Rather, antiviral medications are important for controlling outbreaks and preventing the spread of infection, especially in people at high risk for developing flu complications.

The antiviral medications zanamivir and oseltamivir are used to prevent and treat influenza A and B infections. They can lessen the severity and shorten the duration of flu symptoms.⁵ Amantadine and rimantadine help prevent and treat the flu caused by influenza A (but not influenza B) infection.

Medication Choices

Two types of antiviral medications can treat influenza infections:

Amantadine or rimantadine (used to prevent or treat influenza A infections)

Zanamivir or oseltamivir (used to treat and prevent influenza A and B infections)

What To Think About

Most people do not need antiviral medications. They recover from influenza without developing complications.

However, given how sick most people are when they have the flu, some people may choose to take medication even if they are at low risk for complications.

The U.S. Centers for Disease Control and Prevention (CDC) recommends using amantadine or rimantadine to control outbreaks of influenza A among people who:

• Have a high risk for flu complications.
• Did not get a flu shot and who are in contact with people at risk, such as nursing home and hospital staff.

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.

Influenza cannot be treated with surgery.

No other treatment is available for influenza.

Organization

• Fever, Age 3 and Younger
• Fever, Age 4 and Older

Citations

1. 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.

2. Centers for Disease Control and Prevention (2004). Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR, 53(RR-6): 1–44. Also available online: http://www.cdc.gov/mmwr/PDF/RR/RR5306.pdf. [Erratum: (2004). MMWR, 53(RR-6): 743. Also available online: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5332a8.htm.]

3. Centers for Disease Control and Prevention (2002). Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR, 51(RR-03): 1–31.

4. Uyeki T, Winquist A (2001). Influenza. Clinical Evidence (6): 550–556.

5. Hayden FG, et al. (2000). Inhaled zanamivir for the prevention of influenza in families. New England Journal of Medicine, 343(18): 1282–1289.

Author:	Sydney Youngerman-Cole, RN, BSN, RNC	Last Updated July 17, 2006
Medical Review:	Adam Husney, MD - Family Medicine W. David Colby, MSc, MD, FRCPC - Infectious Disease
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