Also Known As
Flu
Avian Flu
Swine Flu
H1N1
Influenza A
Influenza B
H3N2
H5N1
H7N9
This article was last reviewed on
This article waslast modified on
February 28, 2018.
What is influenza?

Influenza, commonly called the flu, is a viral respiratory infection that spreads from person to person through coughing, sneezing, and contact with contaminated surfaces. 

In colder climates, influenza is seasonal. In the United States, flu season can begin as early as October and end as late as May. In warmer regions of the world, it may be present year-round. During each flu season, there are multiple strains of influenza present, but typically one or two strains predominate as that year's "seasonal flu."

Influenza can cause a spectrum of illness, from mild to severe, and sometimes it can be fatal. According to the Centers for Disease Control and Prevention (CDC), millions of Americans get the flu in the U.S. each year and thousands of people are hospitalized. Seasonal flu-related deaths are difficult to estimate and vary from year to year. (See the CDC’s FluView for the latest estimates.)

With the usual seasonal flu, the highest infection rates are seen in the very young, women who are pregnant, and people with weakened (compromised) immune systems or pre-existing lung disease, but the elderly are especially vulnerable. Most influenza-related deaths and hospitalizations occur in those over 65 years of age.

Estimating the actual number of flu cases is difficult because many of those who get the flu do not seek medical treatment and, of those who do, only a small number are tested. Testing is more common for people who are hospitalized, but overall, laboratory-confirmed cases of influenza only represent a small percentage of those in a community who actually have the flu.

Three types of influenza virus can infect humans. Types A and B cause annual flu pandemics and most epidemics. Type C can cause mild respiratory illness and is not thought to cause epidemics.

Influenza A and B viruses change over time; they can mutate (antigenic drift) to develop different strains. Seasonal influenza strains can undergo a series of genetic changes (antigenic shift) so that people no longer have immunity from prior infections or vaccination. Usually a single strain of influenza virus A will predominate during a particular flu season, although there may be a mixture of A and B causing outbreaks in the community at the same time.

When a large number people are susceptible to a virus, an influenza epidemic may occur. Of particular concern is the spread of a virus globally, resulting in a pandemic. Influenza A can undergo a major genetic change that can make a virus strain much more lethal and/or easier to transmit.

Flu vaccines that are developed each year to prevent flu infection are based upon data as to which strains are likely to be most common that season. They usually contain weakened forms of the virus (attenuated) or inactivated virus targeting two influenza A strains and one influenza B strain.

Different strains of influenza viruses cause illness in humans and in many animals, including birds, pigs (swine), dogs, whales, horses, and seals. Human influenza strains pass easily from person to person, but most strains of animal influenza rarely infect people. When they do, it is almost exclusively when there is significant close animal contact, such as a person who raises chickens or pigs, or at state fairs. Those infections do not usually pose a risk for person-to-person transmission.

The ongoing worry of the world's medical communities is that an influenza strain that is infecting animals, such as birds or pigs, will mutate sufficiently that it will cause serious illness and death in humans (who have no protective antibodies against it) and that it will become a strain that is transmitted easily from human to human.

For example, during the 2009-2010 flu season, there was a seasonal influenza A strain as well as a new strain identified as 2009 H1N1 influenza A (sometimes called swine flu). According to the CDC, this new strain caused the majority of influenza infections in 2009-2010, and the World Health Organization (WHO) declared the 2009 H1N1 virus a pandemic since it caused multiple influenza outbreaks throughout the world at the same time.

Accordion Title
About Influenza
  • Types

    As discussed in the previous section, two types of influenza virus, types A and B, commonly infect humans and pass easily from person to person, causing yearly flu epidemics. Some influenza viruses infect animals, including birds and pigs (swine), but these only rarely infect humans and are not easily spread from person to person. Most often, there must be close contact for the virus to be transmitted from animal to human.

    Influenza type A viruses are further grouped into numerous subtypes and they are named using the designations H and N, based on two protein antigens on the virus' surface: hemagglutinin (H) and neuraminidase (N). The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2.

    In most cases, the specific name of the influenza virus is only relevant to the medical community and those charged with influenza surveillance. However, in recent years there has been news and focus on, first, avian (bird) flu and then H1N1 (swine) flu.

    Other parts of the influenza strain's formal name include the type of influenza, where it was first isolated, the strain number, and the year it was first isolated from the host. For instance, the predominant strain during the 2003-2004 flu season was influenza A/Fujian/411/2002 (H3N2).

    Avian (Bird) Flu
    Strains of influenza A occur in wild aquatic birds like seagulls and ducks worldwide. They can infect domesticated poultry like geese and chickens. They don't normally infect humans, but they have caused some sporadic cases. Public health officials are concerned that the viruses may change and develop the ability to easily infect people and spread among people, causing a pandemic.

    • Asian highly pathogenic avian influenza A (HPAI), subtype H5N1 was first detected in geese in China in 1996 and first found in humans during a Hong Kong poultry outbreak in 1997. It has since spread to Africa, Asia, Middle East, and Europe. In 2014, Canada reported the first human case of H5N1 infection. It has not yet been detected in birds or humans in the U.S. So far, this strain of influenza has remained mainly a bird-to-human infection, but its fatality rate in humans is much higher than the seasonal flu.
    • A new avian influenza A virus, H7N9, was reported infecting birds and people in the spring of 2013 in China. It caused 135 infections that spring and 44 deaths. So far, there is no evidence of extensive person-to-person spread of H7N9, though there have been some isolated cases under rare circumstances. It has not been detected in birds or humans in the U.S.


    Swine Flu (Variant Viruses)
    Swine flu is a type A influenza virus that causes a respiratory infection in pigs. Occasionally, this type of virus may cause infections in humans. Most cases occur in children and teens younger than age 18, according to the Centers for Disease Control and Prevention (CDC), and usually with direct or indirect contact with pigs. These are called variant viruses and are denoted with a "v" after the subtype name. Some variant viruses that have caused infections in humans include H1N1v, H3N2v, and H1N2v.

    • In spring of 2009, a new influenza A (H1N1) virus emerged and caused the first influenza pandemic in 40 years, meaning it spread worldwide and affected many people. The 2009 H1N1 flu virus was originally called "swine flu" because it was similar to a virus found in pigs. It is now known to be a combination of human, swine, and avian flu genes and may cause seasonal influenza infections throughout the world. It was first reported in Mexico and the U.S. as a new influenza A, H1N1 virus.

      The 2009-2010 flu season was the most lethal pandemic in recent history and it had a greater effect on those younger than 65. Between April and December 2009, about 90% of influenza-related hospitalizations and 88% of estimated deaths were in people younger than 65. The CDC estimates that between April 2009 and February 13, 2010, between 42 million and 86 million cases of 2009 H1N1 occurred in the U.S., with between 188,000 and 389,000 H1N1-related hospitalizations and between 8,520 and 17,620 2009 H1N1-related deaths.

      For comparison, the 1918-1919 H1N1 influenza A pandemic infected about one-third of the world's population (an estimated 500 million people) and killed an estimated 20 to 50 million people, with more than 500,000 deaths in the U.S. alone.

    • Another swine influenza virus, H3N2v, is a common respiratory infection found in pigs. In 2012, a jump in the number of cases in humans, particularly in the Midwest, prompted the CDC to alert healthcare practitioners to the growing number. Most of the cases among humans were in children with direct contact with pigs on farms or at county fairs.

      The strain that was circulating within communities had acquired a gene called the matrix (M) gene from the H1N1 virus that caused the 2009 pandemic in humans. The presence of this M gene in the genetic material of the influenza A H3N2v virus concerns epidemiologists, according to the CDC, because it may increase transmissibility of the virus from pigs to people and among people.

  • Signs and Symptoms

    For most people, the seasonal flu is a moderate illness that causes symptoms such as:

    • Fatigue
    • Fever, chills
    • Stuffy nose
    • Sore throat
    • Headache
    • Muscles aches
    • Weakness
    • Cough
    • With some strains, diarrhea and vomiting


    These symptoms may also be seen with a variety of other conditions and seasonal viral infections.

    Influenza, however, can be severe and lead to complications such as viral pneumonia or secondary bacterial pneumonia (see Pneumonia) in the very young, the elderly, in those who are pregnant, and in those with underlying conditions such as asthma, lung disease, heart disease, diabetes, kidney disease, liver disease, and in those with weakened (compromised) immune systems as with cancer or HIV/AIDS.

    The average time from first exposure to illness (incubation period) for influenza is about two days, and it ranges from one to four days. Uncomplicated symptoms usually last about three to seven days. People shed virus and are infectious about a day before symptoms emerge and then for about five to ten days, or until about 24 hours after their fever ends. Children and those with compromised immune systems may be infectious for longer.

  • Tests

    Many individuals with the flu are not tested. They either do not seek medical treatment or their healthcare practitioner makes the diagnosis of probable-flu based upon the person's symptoms, the fact that it is the flu season, and based upon the presence of influenza in the community. This is partially because rapid influenza tests will not detect every case of influenza and partially because more definitive tests take too long to be useful to guide treatment. If it is to be useful in helping healthcare practitioners make decisions about treatment, testing must be done within 3 to 4 days of the onset of symptoms.

    Testing is primarily performed for those who are seriously ill (hospitalized) and for those who are at risk for a severe case of influenza or for complications.

    Laboratory tests
    Several different types of influenza tests may be used to detect an infection. These include:

    • Rapid influenza tests—these generally provide results relatively quickly. Some have been approved to be used at the point of care and may be performed in a healthcare practitioner's office or at a hospital patient's bedside. Some types can only detect influenza A; others can detect both A and B but not distinguish between the two. Still others can detect and distinguish between influenza A and B, and some are further able to differentiate between the strains of influenza A, such as H1N1.
      • Rapid influenza diagnostic test antigen detection – used to detect influenza antigens in a respiratory sample. These have limited sensitivity and will generally detect 40-70% of influenza cases.
      • Molecular tests (e.g., Real-time Reverse Transcription Polymerase Chain Reaction RT-PCR) – these tests detect the influenza viral genetic material (RNA) in a sample. They are generally more sensitive and specific for the influenza virus than rapid antigen tests. They will identify 66% to 100% of influenza cases, depending on the test used.
    • Viral culture—the availability of viral cultures is decreasing as most laboratories adopt molecular assays to detect viral infections. Viral cultures are costly and more difficult to perform and take up to 3 to 10 days to provide a result, which makes them less clinically useful for evaluating an individual. Culture can detect influenza A and B and determine the strain and subtype of influenza, which helps formulate the influenza vaccine for the following year and identify emerging strains that could lead to a pandemic. Cultures can detect other respiratory viruses that may be causing someone's symptoms.


    For more on these, see the article on Influenza Tests.

    A healthcare practitioner may order other lab tests in conjunction with an influenza test:


    Non-laboratory tests

    • Chest X-ray – sometimes done to evaluate the lungs of a person with symptoms of pneumonia
  • Treatment and Prevention

      Treatment

      Many people who do get influenza are only moderately ill and do not require medical treatment. They can receive supportive care that includes drinking plenty of fluids, getting enough bed rest, and taking over-the-counter pain- and fever-reducing medications to relieve symptoms until the infection resolves.

      Antiviral medications are available, however, to treat those who have influenza. They should be started as soon as possible after the emergence of symptoms. When administered within the first 48 hours of the start of symptoms, they can lessen the severity and duration of the illness.

      The CDC recommends treatment for those with confirmed or suspected influenza who have severe, complicated, or progressive illness or who are hospitalized, and for those at risk of severe complications. People at high risk may be treated before they become ill (antiviral chemoprophylaxis) if they have been in close contact with someone who has influenza.

      Those who develop secondary complications, such as bacterial pneumonia, will also require antibiotics.

      Prevention

      • The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months old and older get a flu shot each season. In particular, people who are at high risk of complications from the flu should receive the vaccine. These include young children, the elderly, residents of nursing homes and other long-term care facilities, pregnant women, and people with pre-existing conditions such as asthma, COPD, heart disease, and liver or kidney disorders. See the CDC page on Key Facts About Seasonal Flu Vaccine to learn more.
      • You can also take actions to minimize spread of the flu. Thorough and frequent hand-washing, cleaning potentially contaminated surfaces, coughing and sneezing into tissues, and, when ill, staying home and limiting contact with other people are all important actions that help prevent influenza from spreading.
    View Sources

    NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used. To access online sources, copy and paste the URL into your browser.

    Sources Used in Current Review

    (December 29, 2016) Centers for Disease Control and Prevention. Influenza. Available online at https://www.cdc.gov/flu/. Accessed January 2017.

    (December 20, 2013) Centers for Disease Control and Prevention. Avian Influenza A (H7N9) Virus. Available online at https://www.cdc.gov/flu/avianflu/h7n9-virus.htm. Accessed January 2017.

    (April 27, 2015) Centers for Disease Control and Prevention. Highly Pathogenic Asian Avian Influenza A (H5N1) Virus. Available online at https://www.cdc.gov/flu/avianflu/h5n1-virus.htm. Accessed January 2017.

    (August 23, 2016) Centers for Disease Control and Prevention. Information on Swine Influenza/Variant Influenza Virus. Available online at https://www.cdc.gov/flu/swineflu/index.htm. Accessed January 2017.

    (October 26, 2016) Centers for Disease Control and Prevention. Influenza Signs and Symptoms and the Role of Laboratory Diagnostics. Available online at https://www.cdc.gov/flu/professionals/diagnosis/labrolesprocedures.htm. Accessed December 2016.

    (September 23, 2016) Centers for Disease Control and Prevention. Influenza Virus Testing Methods. Available online at https://www.cdc.gov/flu/professionals/diagnosis/table-testing-methods.htm. Accessed December 2016.

    (October 25, 2016) Centers for Disease Control and Prevention. Rapid Diagnostic Testing for Influenza: Information for Health Care Professionals. Available online at https://www.cdc.gov/flu/professionals/diagnosis/rapidclin.htm. Accessed December 2016.

    (October 25, 2016) Centers for Disease Control and Prevention. Guidance for Clinicians on the Use of RT-PCR and Other Molecular Assays for Diagnosis of Influenza Virus Infection. Available online at https://www.cdc.gov/flu/professionals/diagnosis/molecular-assays.htm. Accessed December 2016.

    (October 14, 2016) Centers for Disease Control and Prevention. Key Facts About Seasonal Flu Vaccine. Available online at https://www.cdc.gov/flu/protect/keyfacts.htm. Accessed January 2017.

    (July 19, 2016) National Institute of Allergy and Infectious Diseases. Influenza. Available online at https://www.niaid.nih.gov/diseases-conditions/influenza. Accessed December 2016.

    Sources Used in Previous Reviews

    (Updated 2009 December 24). H1N1 Flu: Diagnosis and Lab Testing. Centers for Disease Control and Prevention, [On-line information]. Available online at http://www.cdc.gov/h1n1flu/diagnosis/. Accessed March 2010.

    Chang, H. (2009 November 4). Influenza. JAMA. 2009;302(17):1926 [On-line information]. PDF available for download at http://jama.ama-assn.org/cgi/reprint/302/17/1926.pdf. Accessed March 2010.

    (Updated 2009 December 7). Updated Interim Recommendations for the Use of Antiviral Medications in the Treatment and Prevention of Influenza for the 2009-2010 Season. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/recommendations.htm. Accessed March 2010.

    (2009 September 29). Questions & Answers, Influenza Diagnostic Testing During the 2009-2010 Flu Season. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/diagnostic_testing_public_qa.htm. Accessed March 2010.

    (2010 February 10). Questions & Answers, 2009 H1N1 Flu ("Swine Flu") and You. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/qa.htm. Accessed March 2010.

    Derlet, R. et. al. (Updated 2009 December 8). Influenza. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/219557-overview. Accessed March 2010.

    Bronze, M. S. (Updated 2010 January 27). H1N1 Influenza (Swine Flu) eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/1807048-overview. Accessed March 2010.

    Katz, M. et. al. (2009 March 13). Use of Rapid Tests and Antiviral Medications for Influenza Among Primary Care Providers in the United States. Medscape Today from Influenza Resp Viruses. 2009;3(1):29-35 [On-line information]. Available online at http://www.medscape.com/viewarticle/588778. Accessed March 2010.

    (2009 November 13). Fact Sheet for Healthcare Providers: Interpreting ELITech Molecular Diagnostics 2009-H1N1 Influenza A virus Real-Time RT-PCR Test Results ARUP Laboratories [On-line information]. PDF available for download at http://www.arup-lab.com/Testing-Information/resources/HotLines/H1N1-HealthcareProviders-InterpretingTestResults.pdf. Accessed March 2010.

    Hillyard, D. et. al. (Updated 2010 February). Influenza Virus. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/InfluenzaVirus.html?client_ID=LTD. Accessed March 2010.

    Mayo Clinic Staff (2009 September 30). H1N1 flu (swine flu). MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/swine-flu/DS01144. Accessed March 2010.

    Mayo Clinic Staff (2009 September 11). Influenza (flu). MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/influenza/DS00081. Accessed March 2010.

    (2009 August 10). Interim Guidance for the Detection of Novel Influenza A Virus Using Rapid Influenza Diagnostic Tests. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/guidance/rapid_testing.htm. Accessed March 2010.

    (Updated 2010 March 8). CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April 2009 – February 13, 2010. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/estimates_2009_h1n1.htm. Accessed March 2010.

    Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1225, 1578.

    Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 729.

    Reviewed January 2012.Seasonal Influenza. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/swineflu/variant.htm. Accessed November 2013.

    Updated August 2012. Avian Influenza A/ (H5N1) Cumulative Number of Confirmed Human Deaths. Kaiser Family Foundation. Available online at http://kff.org/global-indicator/avian-flu-deaths/. Accessed November 2013.

    Updated Feb. 16, 2012. Charles E. Stager. Diagnostic Influenza Tests. Medscape. Available online at http://emedicine.medscape.com/article/2053517-overview. Accessed November 2013.

    Updated August 2013. Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm. Accessed November 2013.

    Reviewed December 2011. Influenza symptoms and the role of laboratory diagnostics. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/professionals/diagnosis/labrolesprocedures.htm. Accessed November 2013.