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Home > Health Library > Influenza (Seasonal 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.
The flu is usually caused by influenza viruses A and B. There are different strains of the flu virus every year.
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.
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.
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.
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:
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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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.
The virus is spread from person to person through:
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.
The symptoms of influenza (flu) appear suddenly and often include:
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.
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:
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 having severe symptoms and complications is higher for:
Call 911 or other emergency services if:
Call your doctor if:
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.
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:
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.
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:
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.
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:
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.
Increase your chance of staying healthy by:
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.
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:
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.
Two antiviral medicines, oseltamivir and zanamivir, can treat and prevent influenza A and B infections.
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:
Nichol KL, et al. (2007). Effectiveness of influenza vaccine in the community-dwelling elderly. New England Journal of Medicine, 357(14): 1373–1381.
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.
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.
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.
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.
Current as of:
July 6, 2021
Author: Healthwise StaffMedical Review: E. Gregory Thompson MD - Internal MedicineJohn Pope MD - PediatricsAdam Husney MD - Family MedicineKathleen Romito MD - Family MedicineChristine Hahn MD - Epidemiology
Current as of: July 6, 2021
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
To learn more about Healthwise, visit Healthwise.org.
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