Influenza

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.

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, or types, 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. The best time to get the vaccine is in October or November, just before the start of flu season. You can get the vaccine as a shot or in a spray that you breathe in through your nose.

Almost anyone over 6 months old can have the 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

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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?

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.

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 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.¹
• Women who will be in their second or third trimester of pregnancy during the flu season.¹
• 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 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.

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 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.

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. For more information, see:

Should I take antiviral medicine for the flu?

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?) (FluMist) prevents flu infection and its complications in most people. FluMist is approved for use by healthy people who are 2 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 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 shot.

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.

To help you decide if the flu shot is right for you, see:

Should I get a flu shot?

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.

Prevention

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.³

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.

For help deciding if the flu shot is right for you, see:

Should I get a flu shot?

Most healthy people ages 2 through 49 years can choose to get the nasal spray form (What is a PDF document?) of the vaccine (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 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 medicines to prevent the flu

Two antiviral medicines (zanamivir and oseltamivir) can 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 the flu shot 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.⁴ These medicines have not worked against most types of the flu virus. Amantadine and rimantadine do not protect against influenza B. It is important to talk with your doctor about the medicine that is best for you.

For more information, see:

Should I take antiviral medicine for the flu?

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, 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.
• To help clear a stuffy nose, breathe moist air from a hot shower or from a sink filled with hot water.
• 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. Do not give cough and cold medicines to a child younger than 2 unless your child's doctor has told you to. If your child's doctor tells you to give a medicine, be sure to follow what he or she tells you to do.
• 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.

Medications

Some antiviral medicines 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 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.⁵ 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.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.

For more information, see:

Should I take antiviral medicine for the flu?

Medication Choices

Two types of antiviral medicines can treat influenza infections:

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

Amantadine or rimantadine (used to prevent or treat influenza A 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.

Surgery

Influenza cannot be treated with surgery.

Other Treatment

No other treatment is available for influenza.

Other Places To Get Help

Organizations

Related Information

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

References

Citations

1. Centers for Disease Control and Prevention (2008). Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR, 57(RR-7): 1-60. Also available online: http://www.cdc.gov/mmwr/PDF/rr/rr5707.pdf.

2. Jefferson T (2007). Influenza, search date April 2007. Online version of BMJ Clinical Evidence. Also available online: 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. 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.

5. 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.

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