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Influenza, commonly known as the flu, is an infectious disease caused by an influenza virus.[1]


The most common symptoms include: high fever, runny nose, sore throat, muscle pains, headache, coughing, sneezing, and feeling tired.[1][2]

Timing of symptoms

These symptoms typically begin two days after exposure to the virus, and most last less than a week.[1] The cough, however, may last for more than two weeks.[1]

Symptoms in children

In children, there may be diarrhea and vomiting, although [3] these symptoms are more commonly caused by the unrelated illness, gastroenteritis (otherwise known as the stomach, or 24 hour flu).[3]


Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems, such as asthma, or heart failure.[4][2]


Influenza viruses are classified into four types. Types A, B, and C, affect humans.[4][5] Type D does not, but is believed to have the potential for it.[5][6]

Airborne spread

Usually, the virus is spread through the air, from coughs or sneezes, [1] over relatively short distances.[7]

Surface spread

It can also be spread by touching surfaces contaminated by the virus, and then touching the mouth or eyes.[2][7]

Infectious period

A person may be infectious to others both before, and during, the time they are showing symptoms.[2]


The infection may be confirmed by testing the throat, sputum, or nose for the virus[4] with a rapid test, although people may still have the infection even if the results are negative.[4]


Frequent hand washing, or wearing a surgical mask, reduces the risk of viral spread.[8][8]


Yearly vaccinations against influenza are recommended by the World Health Organization, for those at high risk.[1]

Vaccine effectiveness

The vaccine is usually effective against three or four types of influenza,[1] is usually well-tolerated, [1] but only effective for a single season, since the virus evolves rapidly.[1]

Antiviral drugs

Antiviral drugs such as the neuraminidase inhibitor oseltamivir, have been used to treat influenza,[1] but the benefit to otherwise healthy patients does not appear to be greater than the risks, [9] and no benefit has been found in those with other health problems.[9][10]


Influenza spreads around the world in yearly outbreaks, resulting in three to five million cases of severe illness, and 250 to 500 thousand deaths.[1] About 20% of unvaccinated children, and 10% of unvaccinated adults are infected each year.[11]


In the northern and southern parts of the world, outbreaks occur mainly in the winter, while around the equator, outbreaks may occur at any time of the year.[1]

Highest risk of death

Death occurs mostly in the young, the old, and those with other health problems.[1] Larger outbreaks, known as pandemics, are less frequent.[4]


In the 20th century, three influenza pandemics occurred.

Spanish flu

The Spanish influenza in 1918 that resulted in 50 million deaths,

Asian influenza

The Asian influenza in 1957 that killed two million,

Hong Kong influenza

and Hong Kong influenza in 1968, that resulted in one million deaths.[12]

Swine flu

The World Health Organization declared an outbreak of a new type of influenza A, called H1N1, or swine flu, that resulted in a pandemic during 2009, and 2010.[13]

Other animals

Influenza may also affect other animals, including pigs, horses, and birds.[14]


  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 "Influenza (Seasonal) Fact sheet N°211". March 2014. Archived from the original on 30 November 2014. Retrieved 25 November 2014.
  2. 2.0 2.1 2.2 2.3 "Key Facts about Influenza (Flu) & Flu Vaccine". 9 September 2014. Archived from the original on 2 December 2014. Retrieved 26 November 2014.
  3. 3.0 3.1 Duben-Engelkirk PG, Engelkirk J (2011). Burton's microbiology for the health sciences (9th ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. p. 314. ISBN 978-1-60547-673-5.
  4. 4.0 4.1 4.2 4.3 4.4 Longo DL (2012). "Chapter 187: Influenza". Harrison's principles of internal medicine (18th ed.). New York: McGraw-Hill. ISBN 978-0-07-174889-6.
  5. 5.0 5.1 "Types of Influenza Viruses Seasonal Influenza (Flu)". CDC. 27 September 2017. Retrieved 28 September 2018.
  6. Shuo Su; Xinliang Fu; Gairu Li; Fiona Kerlin; Michael Veit (25 August 2017). "Novel Influenza D virus: Epidemiology, pathology, evolution and biological characteristics". Virulence. 8 (8): 1580–91. doi:10.1080/21505594.2017.1365216. PMC 5810478. PMID 28812422.
  7. 7.0 7.1 Brankston G, Gitterman L, Hirji Z, Lemieux C, Gardam M (April 2007). "Transmission of influenza A in human beings". Lancet Infect Dis. 7 (4): 257–65. doi:10.1016/S1473-3099(07)70029-4. PMID 17376383.
  8. 8.0 8.1 Jefferson T, Del Mar CB, Dooley L, et al. (2011). "Physical interventions to interrupt or reduce the spread of respiratory viruses" (PDF). Cochrane Database Syst Rev (7): CD006207. doi:10.1002/14651858.CD006207.pub4. PMID 21735402.
  9. 9.0 9.1 Michiels B, Van Puyenbroeck K, Verhoeven V, Vermeire E, Coenen S (2013). "The value of neuraminidase inhibitors for the prevention and treatment of seasonal influenza: a systematic review of systematic reviews". PLOS One. 8 (4): e60348. Bibcode:2013PLoSO...860348M. doi:10.1371/journal.pone.0060348. PMC 3614893. PMID 23565231.
  10. Ebell MH, Call M, Shinholser J (April 2013). "Effectiveness of oseltamivir in adults: a meta-analysis of published and unpublished clinical trials". Family Practice. 30 (2): 125–33. doi:10.1093/fampra/cms059. PMID 22997224.
  11. Somes MP, Turner RM, Dwyer LJ, Newall AT (May 2018). "Estimating the annual attack rate of seasonal influenza among unvaccinated individuals: A systematic review and meta-analysis". Vaccine. 36 (23): 3199–3207. doi:10.1016/j.vaccine.2018.04.063. PMID 29716771.
  12. "Ten things you need to know about pandemic influenza". World Health Organization. 14 October 2005. Archived from the original on 8 October 2009. Retrieved 26 September 2009.
  13. Chan, Margaret (11 June 2009). "World now at the start of 2009 influenza pandemic". World Health Organization. Archived from the original on 12 June 2009. Retrieved 12 June 2009.
  14. Palmer SR (2011). Oxford textbook of zoonoses : biology, clinical practice, and public health control (2. ed.). Oxford u.a.: Oxford Univ. Press. p. 332. ISBN 978-0-19-857002-8.