Influenza, commonly known as the flu, is an infectious disease caused by an influenza virus.[1] Symptoms can be mild to severe.[4] The most common symptoms include: high fever, runny nose, sore throat, muscle pains, headache, coughing, sneezing, and feeling tired.[1] 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] In children, there may be diarrhea and vomiting, but these are not common in adults.[5] Diarrhea and vomiting occur more commonly in gastroenteritis, which is an unrelated disease and sometimes inaccurately referred to as "stomach flu" or the "24-hour flu".[5] Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.[2][4]

Three of the four types of influenza viruses affect humans: Type A, Type B, and Type C.[2][6] Type D has not been known to infect humans, but is believed to have the potential to do so.[6][7] Usually, the virus is spread through the air from coughs or sneezes.[1] This is believed to occur mostly over relatively short distances.[8] It can also be spread by touching surfaces contaminated by the virus and then touching the mouth or eyes.[4][8] A person may be infectious to others both before and during the time they are showing symptoms.[4] The infection may be confirmed by testing the throat, sputum, or nose for the virus.[2] A number of rapid tests are available; however, people may still have the infection even if the results are negative.[2] A type of polymerase chain reaction that detects the virus's RNA is more accurate.[2]

Frequent hand washing reduces the risk of viral spread.[3] Wearing a surgical mask is also useful.[3] Yearly vaccinations against influenza are recommended by the World Health Organization for those at high risk.[1] The vaccine is usually effective against three or four types of influenza.[1] It is usually well-tolerated.[1] A vaccine made for one year may not be useful in the following year, since the virus evolves rapidly.[1] Antiviral drugs such as the neuraminidase inhibitor oseltamivir, among others, have been used to treat influenza.[1] The benefit of antiviral drugs in those who are otherwise healthy do not appear to be greater than their risks.[9] No benefit has been found in those with other health problems.[9][10]

Influenza spreads around the world in yearly outbreaks, resulting in about three to five million cases of severe illness and about 250,000 to 500,000 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] Death occurs mostly in the young, the old, and those with other health problems.[1] Larger outbreaks known as pandemics are less frequent.[2] In the 20th century, three influenza pandemics occurred: Spanish influenza in 1918 (~50 million deaths), Asian influenza in 1957 (two million deaths), and Hong Kong influenza in 1968 (one million deaths).[12] The World Health Organization declared an outbreak of a new type of influenza A/H1N1 to be a pandemic in June 2009.[13] 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 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 Influenza (Seasonal) Fact sheet N°211”. March 2014. 
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Longo, Dan L. (2012). „Chapter 187: Influenza”. Harrison's principles of internal medicine. (ed. 18th). New York: McGraw-Hill. ISBN 978-0-07-174889-6. 
  3. 3.0 3.1 3.2 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. 
  4. 4.0 4.1 4.2 4.3 Key Facts about Influenza (Flu) & Flu Vaccine”. 9 September 2014. Archived from the original on 2 December 2014. Accesat la 26 November 2014. 
  5. 5.0 5.1 Duben-Engelkirk, Paul G.; Engelkirk, Janet (2011). Burton's microbiology for the health sciences (ed. 9th). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. p. 314. ISBN 978-1-60547-673-5. 
  6. 6.0 6.1 Types of Influenza Viruses Seasonal Influenza (Flu)” (în en-us). 27 September 2017. Accesat la 28 September 2018. 
  7. 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 5810478Freely accessible. PMID 28812422. 
  8. 8.0 8.1 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. 
  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 3614893Freely accessible. 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. 
  13. Chan, Margaret (11 June 2009). „World now at the start of 2009 influenza pandemic”. World Health Organization. 
  14. Oxford textbook of zoonoses : biology, clinical practice, and public health control (ed. 2.). Oxford u.a.: Oxford Univ. Press. 2011. p. 332. ISBN 978-0-19-857002-8. 
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