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{{hatnote|"Flu" and "Grippe" redirect here. For other uses, see Flu (disambiguation) and Grippe (disambiguation). Not to be confused with Haemophilus influenzae.}}{{pp-semi|small=yes}}{{pp-move-indef}}{{short description|infectious disease}}{{Use dmy dates|date=December 2018}}

Influenza, commonly known as the flu, is an infectious disease caused by an influenza virus. Symptoms can be mild to severe.WEB, Key Facts About Influenza (Flu),weblink Centers for Disease Control and Prevention (CDC), 26 November 2014, 9 September 2014, live,weblink" title="">weblink 2 December 2014, The most common symptoms include: high fever, runny nose, sore throat, muscle pains, headache, coughing, sneezing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be diarrhea and vomiting, but these are not common in adults. 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".BOOK, Duben-Engelkirk, Paul G., Engelkirk, Janet, vanc, Burton's microbiology for the health sciences, 2011, Wolters Kluwer Health/Lippincott Williams & Wilkins, Philadelphia, 978-1-60547-673-5, 314, 9th,weblink Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.Three of the four types of influenza viruses affect humans: Type A, Type B, and Type C.BOOK, Longo, Dan L., vanc, Harrison's principles of internal medicine, 2012, McGraw-Hill, New York, 978-0-07-174889-6, 18th, Chapter 187: Influenza, WEB, Types of Influenza Viruses Seasonal Influenza (Flu),weblink Centers for Disease Control and Prevention (CDC), 28 September 2018, 27 September 2017, Type D has not been known to infect humans, but is believed to have the potential to do so.JOURNAL, Su S, Fu X, Li G, Kerlin F, Veit M, Novel Influenza D virus: Epidemiology, pathology, evolution and biological characteristics, Virulence, 8, 8, 1580–91, 25 August 2017, 28812422, 5810478, 10.1080/21505594.2017.1365216, Usually, the virus is spread through the air from coughs or sneezes. This is believed to occur mostly over relatively short distances.JOURNAL, Brankston G, Gitterman L, Hirji Z, Lemieux C, Gardam M, Transmission of influenza A in human beings, Lancet Infect Dis, 7, 4, 257–65, April 2007, 17376383, 10.1016/S1473-3099(07)70029-4, It can also be spread by touching surfaces contaminated by the virus and then touching the mouth or eyes. A person may be infectious to others both before and during the time they are showing symptoms. The infection may be confirmed by testing the throat, sputum, or nose for the virus. A number of rapid tests are available; however, people may still have the infection even if the results are negative. A type of polymerase chain reaction that detects the virus's RNA is more accurate.Frequent hand washing reduces the risk of viral spread.JOURNAL, Jefferson T, Del Mar CB, Dooley L, Ferroni E, Al-Ansary LA, Bawazeer GA, van Driel ML, Nair S, Jones MA, Thorning S, Conly JM, 6, Physical interventions to interrupt or reduce the spread of respiratory viruses, Cochrane Database Syst Rev, 7, CD006207, 2011, 21735402, 10.1002/14651858.CD006207.pub4,weblink Wearing a surgical mask is also useful. Yearly vaccinations against influenza are recommended by the World Health Organization (WHO) for those at high risk, and by the Centers for Disease Control and Prevention (CDC) for those six months of age and older. The vaccine is usually effective against three or four types of influenza. It is usually well-tolerated. A vaccine made for one year may not be useful in the following year, since the virus evolves rapidly. Antiviral drugs such as the neuraminidase inhibitor oseltamivir, among others, have been used to treat influenza. The benefit of antiviral drugs in those who are otherwise healthy do not appear to be greater than their risks.JOURNAL, Michiels B, Van Puyenbroeck K, Verhoeven V, Vermeire E, Coenen S, The value of neuraminidase inhibitors for the prevention and treatment of seasonal influenza: a systematic review of systematic reviews, PLOS ONE, 8, 4, e60348, 2013, 23565231, 3614893, 10.1371/journal.pone.0060348, 2013PLoSO...860348M, No benefit has been found in those with other health problems.JOURNAL, Ebell MH, Call M, Shinholser J, Effectiveness of oseltamivir in adults: a meta-analysis of published and unpublished clinical trials, Family Practice, 30, 2, 125–33, April 2013, 22997224, 10.1093/fampra/cms059, 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.WEB, Influenza (Seasonal), World Health Organization (WHO), 25 November 2014, March 2014,weblink live,weblink" title="">weblink 30 November 2014, About 20% of unvaccinated children and 10% of unvaccinated adults are infected each year.JOURNAL, Somes MP, Turner RM, Dwyer LJ, Newall AT, Estimating the annual attack rate of seasonal influenza among unvaccinated individuals: A systematic review and meta-analysis, Vaccine, 36, 23, 3199–3207, May 2018, 29716771, 10.1016/j.vaccine.2018.04.063, 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. Death occurs mostly in the young, the old, and those with other health problems. Larger outbreaks known as pandemics are less frequent. In the 20th century, three influenza pandemics occurred: Spanish influenza in 1918 (~50{{nbsp}}million deaths), Asian influenza in 1957 (two million deaths), and Hong Kong influenza in 1968 (one million deaths).WEB,weblink World Health Organization (WHO), 14 October 2005, Ten things you need to know about pandemic influenza, 26 September 2009,weblink" title="">weblink 8 October 2009, dead, JOURNAL, Wkly Epidemiol Rec., 2005-12-09, 80, 49–50, 428–31, Ten things you need to know about pandemic influenza (update of 14 October 2005), 16372665, ((World Health Organization)),weblink The World Health Organization declared an outbreak of a new type of influenza A/H1N1 to be a pandemic in June 2009.WEB,weblink World now at the start of 2009 influenza pandemic, World Health Organization (WHO), 11 June 2009, Chan, Margaret, 12 June 2009, live,weblink" title="">weblink 12 June 2009, vanc, Influenza may also affect other animals, including pigs, horses, and birds.BOOK, Palmer SR, Oxford textbook of zoonoses : biology, clinical practice, and public health control, 2011, Oxford Univ. Press, Oxford u.a., 978-0-19-857002-8, 332, 2.,weblink File:Wikipedia-VideoWiki-Influenza.webm|thumb|thumbtime=2:41|upright=1.3|Video summary (script)]]{{TOC limit|3}}

Signs and symptoms {| class"wikitable" style"float:right; text-align:center; width:40%;"

sensitivity (tests)>sensitive symptoms for diagnosing influenza! Symptom: !! sensitivity !! specificity! Fever|25–73%! Cough| 7–29%! Nasal congestion|19–41%
  • All three findings, especially fever, were less sensitive in people over 60 years of age.
(File:Symptoms of influenza.svg|thumb|upright=1.35|Symptoms of influenza,WEB,weblink Centers for Disease Control and Prevention (CDC), Influenza Symptoms,weblink" title="">weblink 1 May 2009, 28 April 2009, 16 November 2007, dead, WEB, Flu Symptoms & Complications, Centers for Disease Control and Prevention (CDC), 26 February 2019,weblink 2019-07-06, with fever and cough the most common symptoms.)Approximately 33% of people with influenza are asymptomatic.JOURNAL, Carrat F, Vergu E, Ferguson NM, Lemaitre M, Cauchemez S, Leach S, Valleron AJ, 6, Time Lines of Infection and Disease in Human Influenza: A Review of Volunteer Challenge Studies, American Journal of Epidemiology, 167, 7, 2008-01-29, 1476-6256, 10.1093/aje/kwm375, 775–785, 18230677, In almost all studies, participants were individually confined for 1 week, See especially Figure 5 which shows that virus shedding tends to peak on day 2 whereas symptoms tend to peak on day 3.Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills and body aches, but fever is also common early in the infection, with body temperatures ranging from 38 to 39{{nbsp}}°C (approximately 100 to 103{{nbsp}}°F).JOURNAL, Suzuki E, Ichihara K, Johnson AM, Natural course of fever during influenza virus infection in children, Clin Pediatr (Phila), 46, 1, 76–79, January 2007, 17164515, 10.1177/0009922806289588, Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worse in their backs and legs.WEB,weblink Influenza (Flu), Merck, 15 March 2008, live,weblink" title="">weblink 17 March 2008, Brenda L., Tesini, September 2018, vanc,

Symptoms of influenza

  • Fever and chills
  • Cough
  • Nasal congestion
  • Runny nose
  • Sneezing
  • Sore throat
  • Hoarseness
  • Earache
  • Muscle pains
  • Fatigue
  • Headache
  • Irritated, watering eyes
  • Reddened eyes, skin (especially face), mouth, throat and nose
  • Petechial rashJOURNAL, Silva ME, Cherry JD, Wilton RJ, Ghafouri NM, Bruckner DA, Miller MJ, Acute fever and petechial rash associated with influenza A virus infection, Clinical Infectious Diseases, 29, 2, 453–54, August 1999, 10476766, 10.1086/520240,
  • In children, gastrointestinal symptoms such as vomiting, diarrhea, and abdominal pain,JOURNAL, Heikkinen T, Influenza in children, Acta Paediatr., 95, 7, 778–84, July 2006, 16801171, 10.1080/08035250600612272, (may be severe in children with influenza B)JOURNAL, Kerr AA, McQuillin J, Downham MA, Gardner PS, Gastric 'flu influenza B causing abdominal symptoms in children, Lancet, 1, 7902, 291–95, 1975, 46444, 10.1016/S0140-6736(75)91205-2,
It can be difficult to distinguish between the common cold and influenza in the early stages of these infections.JOURNAL, Eccles R, Understanding the symptoms of the common cold and influenza, Lancet Infect Dis, 5, 11, 718–25, 2005, 16253889, 10.1016/S1473-3099(05)70270-X, Influenza symptoms are a mixture of symptoms of common cold and pneumonia, body ache, headache, and fatigue. Diarrhea is not usually a symptom of influenza in adults, although it has been seen in some human cases of the H5N1 "bird flu"JOURNAL, Hui DS, Review of clinical symptoms and spectrum in humans with influenza A/H5N1 infection, Respirology, 13 Suppl 1, S10–13, March 2008, 18366521, 10.1111/j.1440-1843.2008.01247.x, and can be a symptom in children.JOURNAL, Richards S, Flu blues, Nurs Stand, 20, 8, 26–27, 2005, 16295596, 10.7748/ns.20.8.26.s29, The symptoms most reliably seen in influenza are shown in the adjacent table.JOURNAL, Call S, Vollenweider M, Hornung C, Simel D, McKinney W, Does this patient have influenza?, JAMA, 293, 8, 987–97, 2005, 10.1001/jama.293.8.987, 15728170, The specific combination of fever and cough has been found to be the best predictor; diagnostic accuracy increases with a body temperature above 38°C (100.4°F).JOURNAL, Monto A, Gravenstein S, Elliott M, Colopy M, Schweinle J, Clinical signs and symptoms predicting influenza infection, Arch Intern Med, 160, 21, 3243–47, 2000, 11088084, 10.1001/archinte.160.21.3243, Two decision analysis studiesJOURNAL, Smith K, Roberts M, Cost-effectiveness of newer treatment strategies for influenza, Am J Med, 113, 4, 300–07, 2002, 10.1016/S0002-9343(02)01222-6, 12361816,, JOURNAL, Rothberg M, Bellantonio S, Rose D, Management of influenza in adults older than 65 years of age: cost-effectiveness of rapid testing and antiviral therapy, Annals of Internal Medicine, 139, 5 Pt 1, 321–29, 2 September 2003, 12965940, 10.7326/0003-4819-139-5_part_1-200309020-00007, suggest that during local outbreaks of influenza, the prevalence will be over 70%. Even in the absence of a local outbreak, diagnosis may be justified in the elderly during the influenza season as long as the prevalence is over 15%.The United States Centers for Disease Control and Prevention (CDC) maintains an up-to-date summary of available laboratory tests.WEB, Centers for Disease Control and Prevention (CDC),weblink Information for Clinicians on Influenza Virus Testing,weblink" title="">weblink 3 May 2009, 1 May 2009, 26 February 2018, live, According to the CDC, rapid diagnostic tests have a sensitivity of 50–75% and specificity of 90–95% when compared with viral culture.WEB,weblink Rapid Diagnostic Testing for Influenza: Information for Clinical Laboratory Directors, Centers for Disease Control and Prevention (CDC), 13 October 2015, 2 February 2016, live,weblink" title="">weblink 16 January 2016, dmy-all, Occasionally, influenza can cause severe illness including primary viral pneumonia or secondary bacterial pneumonia.JOURNAL, Jain S, Kamimoto L, Bramley AM, Schmitz AM, Benoit SR, Louie J, Sugerman DE, Druckenmiller JK, Ritger KA, Chugh R, Jasuja S, Deutscher M, Chen S, Walker JD, Duchin JS, Lett S, Soliva S, Wells EV, Swerdlow D, Uyeki TM, Fiore AE, Olsen SJ, Fry AM, Bridges CB, Finelli L, ((2009 Pandemic Influenza A (H1N1) Virus Hospitalizations Investigation Team)), 6, Hospitalized Patients with 2009 H1N1 Influenza in the United States, April–June 2009, New England Journal of Medicine, 361, 20, 2009-11-12, 0028-4793, 10.1056/nejmoa0906695, 1935–1944, 19815859, Transcript of virtual press conference with Gregory Hartl, Spokesperson for H1N1, and Dr Nikki Shindo, Medical Officer, Global Influenza Programme, World Health Organization {{webarchive|url= |date=29 November 2009 }}, 12 November 2009. The obvious symptom is trouble breathing. In addition, if a child (or presumably an adult) seems to be getting better and then relapses with a high fever, that is a danger sign since this relapse can be bacterial pneumonia.NEWS, Grady, Denise, Report Finds Swine Flu Has Killed 36 Children, The New York Times, 2009-09-03,weblinkweblink" title="">weblink 27 June 2017, live, vanc, Sometimes, influenza may have abnormal presentations, like confusion in the elderly and a sepsis-like syndrome in the young.WEB,weblink Guide for considering influenza testing when influenza viruses are circulating in the community, 20 February 2018, Centers for Disease Control and Prevention (CDC), 30 March 2018,

Emergency warning signs

Signs of dehydration

  • (in infants) Far fewer wet diapers than usualNEWS,weblink The Flu: What To Do If You Get Sick, 9 March 2018, Centers for Disease Control and Prevention (CDC), 29 March 2018,
  • Cannot keep down fluids
  • (in infants) No tears when crying.


Types of virus

File:3D Influenza virus.png|thumb|Structure of the influenza (wikt:virion|virion). The hemagglutinin (HA) and neuraminidase (NA) proteins are shown on the surface of the particle. The viral RNAs that make up the genome are shown as red coils inside the particle and bound to ribonuclearproteins (RNP).]]In virus classification, influenza viruses are RNA viruses that make up four of the seven genera of the family Orthomyxoviridae:BOOK, Kawaoka Y, Influenza Virology: Current Topics, Caister Academic Press, 2006,weblink 978-1-904455-06-6, live,weblink" title="">weblink 9 May 2008, These viruses are only distantly related to the human parainfluenza viruses, which are RNA viruses belonging to the paramyxovirus family that are a common cause of respiratory infections in children such as croup,JOURNAL, Vainionpää R, Hyypiä T, Biology of parainfluenza viruses, Clin. Microbiol. Rev., 7, 2, 265–75, April 1994, 8055470, 358320, 10.1128/CMR.7.2.265, but can also cause a disease similar to influenza in adults.JOURNAL, Hall CB, Respiratory syncytial virus and parainfluenza virus, N. Engl. J. Med., 344, 25, 1917–28, June 2001, 11419430, 10.1056/NEJM200106213442507, A fourth family of influenza viruses was identified in 2016 – Influenza D.JOURNAL, Hause BM, Collin EA, Liu R, Huang B, Sheng Z, Lu W, Wang D, Nelson EA, Li F, 6, 2014, Characterization of a novel influenza virus in cattle and swine: proposal for a new genus in the Orthomyxoviridae family, mBio, 5, 2, e00031–14, 10.1128/mBio.00031-14, 24595369, 3958797, JOURNAL, Collin EA, Sheng Z, Lang Y, Ma W, Hause BM, Li F, 2015, Cocirculation of two distinct genetic and antigenic lineages of proposed influenza D virus in cattle, J Virol, 89, 2, 1036–42, 10.1128/JVI.02718-14, 25355894, 4300623, JOURNAL, Ducatez MF, Pelletier C, Meyer G, 2015, Influenza D virus in cattle, France, 2011–2014, Emerg Infect Dis, 21, 2, 368–71, 10.3201/eid2102.141449, 25628038, 4313661, JOURNAL, Song H, Qi J, Khedri Z, Diaz S, Yu H, Chen X, Varki A, Shi Y, Gao GF, 6, 2016, An open receptor-binding cavity of hemagglutinin-esterase-fusion glycoprotein from newly-identified Influenza D Virus: Basis for its broad cell tropism, PLoS Pathog, 12, 1, e1005411, 10.1371/journal.ppat.1005411, 26816272, 4729479, JOURNAL, Sheng Z, Ran Z, Wang D, Hoppe AD, Simonson R, Chakravarty S, Hause BM, Li F, 6, 2014, Genomic and evolutionary characterization of a novel influenza-C-like virus from swine, Arch Virol, 159, 2, 249–55, 10.1007/s00705-013-1815-3, 23942954, 5714291, JOURNAL, Quast M, Sreenivasan C, Sexton G, Nedland H, Singrey A, Fawcett L, Miller G, Lauer D, Voss S, Pollock S, Cunha CW, Christopher-Hennings J, Nelson E, Li F, 6, 2015, Serological evidence for the presence of influenza D virus in small ruminants, Vet Microbiol, 180, 3–4, 281–85, 10.1016/j.vetmic.2015.09.005, 26414999, 4618254, JOURNAL, Smith DB, Gaunt ER, Digard P, Templeton K, Simmonds P, 2016, Detection of influenza C virus but not influenza D virus in Scottish respiratory samples, J Clin Virol, 74, 50–53, 10.1016/j.jcv.2015.11.036, 26655269, 4710576, The type species for this family is Influenza D virus, which was first isolated in 2011.

Influenzavirus A

This genus has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A. Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics.BOOK,weblink Klenk, Hans-Dieter, Matrosovich, Mikhail, Stech, Jürgen, vanc, 2008, Avian Influenza: Molecular Mechanisms of Pathogenesis and Host Range, Animal Viruses: Molecular Biology, Caister Academic Press, 978-1-904455-22-6, The type A viruses are the most virulent human pathogens among the four influenza types and cause the severest disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:
  • H1N1, which caused Spanish flu in 1918, and Swine Flu in 2009
  • H2N2, which caused Asian Flu in 1957
  • H3N2, which caused Hong Kong Flu in 1968
  • H5N1, which caused Bird Flu in 2004JOURNAL, Wkly Epidemiol Rec., 30 June 2006, 81, 26, 249–57, Epidemiology of WHO-confirmed human cases of avian influenza A(H5N1) infection, 16812929,weblink ((World Health Organization)), JOURNAL, Wkly Epidemiol Rec., 2008-11-14, 83, 46, 415–20, Update: WHO-confirmed human cases of avian influenza A (H5N1) infection, November 2003-May 2008, 19009716, ((World Health Organization)),weblink
  • H7N7, which has unusual zoonotic potentialJOURNAL, Fouchier RA, Schneeberger PM, Rozendaal FW, Broekman JM, Kemink SA, Munster V, Kuiken T, Rimmelzwaan GF, Schutten M, Van Doornum GJ, Koch G, Bosman A, Koopmans M, Osterhaus AD, 6, Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome, Proceedings of the National Academy of Sciences of the United States of America, 101, 5, 1356–61, February 2004, 14745020, 337057, 10.1073/pnas.0308352100, dmy-all, 2004PNAS..101.1356F,
  • H1N2, endemic in humans, pigs and birds
  • H9N2
  • H7N2
  • H7N3
  • H10N7
  • H7N9, responsible for an ongoing epidemic in China and currently has the greatest pandemic potential among the Type A subtypesWEB, Asian Lineage Avian Influenza A(H7N9) Virus,weblink Centers for Disease Control and Prevention (CDC), 10 July 2019, 7 December 2018,
  • H6N1, which only infected one person, who recoveredJOURNAL, Yuan J, Zhang L, Kan X, Jiang L, Yang J, Guo Z, Ren Q, 10.1093/cid/cit479, Origin and Molecular Characteristics of a Novel 2013 Avian Influenza A(H6N1) Virus Causing Human Infection in Taiwan, Clinical Infectious Diseases, 57, 9, 1367–8, 2013-11-01, 23881153, 1537-6591,

Influenzavirus B

File:Influenza nomenclature.svg|thumb|Influenza virus nomenclature (for a Fujian fluFujian fluThis genus has one species, influenza B virus. Influenza B almost exclusively infects humansJOURNAL, Hay AJ, Gregory V, Douglas AR, Lin YP, The evolution of human influenza viruses, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 356, 1416, 1861–70, December 2001, 11779385, 1088562, 10.1098/rstb.2001.0999, and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are the sealJOURNAL, Osterhaus AD, Rimmelzwaan GF, Martina BE, Bestebroer TM, Fouchier RA, Influenza B virus in seals, Science, 288, 5468, 1051–53, May 2000, 10807575, 10.1126/science.288.5468.1051, 2000Sci...288.1051O, and the ferret.JOURNAL, Jakeman KJ, Tisdale M, Russell S, Leone A, Sweet C, Efficacy of 2'-deoxy-2'-fluororibosides against influenza A and B viruses in ferrets, Antimicrobial Agents and Chemotherapy, 38, 8, 1864–67, August 1994, 7986023, 284652, 10.1128/aac.38.8.1864, This type of influenza mutates at a rate 2–3 times slower than type AJOURNAL, Nobusawa E, Sato K, Comparison of the mutation rates of human influenza A and B viruses, Journal of Virology, 80, 7, 3675–78, April 2006, 16537638, 1440390, 10.1128/JVI.80.7.3675-3678.2006, and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible.JOURNAL, Webster RG, Bean WJ, Gorman OT, Chambers TM, Kawaoka Y, Evolution and ecology of influenza A viruses, Microbiological Reviews, 56, 1, 152–79, March 1992, 1579108, 372859, This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.JOURNAL, Zambon MC, Epidemiology and pathogenesis of influenza, The Journal of Antimicrobial Chemotherapy, 44 Suppl B, 90002, 3–9, November 1999, 10877456, 10.1093/jac/44.suppl_2.3,weblinkweblink" title="">weblink dmy-all, live, 23 March 2013,

Influenzavirus C

This genus has one species, influenza C virus, which infects humans, dogs and pigs, sometimes causing both severe illness and local epidemics.JOURNAL, Matsuzaki Y, Sugawara K, Mizuta K, Tsuchiya E, Muraki Y, Hongo S, Suzuki H, Nakamura K, 6, Antigenic and genetic characterization of influenza C viruses which caused two outbreaks in Yamagata City, Japan, in 1996 and 1998, Journal of Clinical Microbiology, 40, 2, 422–29, February 2002, 11825952, 153379, 10.1128/JCM.40.2.422-429.2002, However, influenza C is less common than the other types and usually only causes mild disease in children.JOURNAL, Matsuzaki Y, Katsushima N, Nagai Y, Shoji M, Itagaki T, Sakamoto M, Kitaoka S, Mizuta K, Nishimura H, 6, Clinical features of influenza C virus infection in children, The Journal of Infectious Diseases, 193, 9, 1229–35, May 2006, 16586359, 10.1086/502973, JOURNAL, Katagiri S, Ohizumi A, Homma M, An outbreak of type C influenza in a children's home, The Journal of Infectious Diseases, 148, 1, 51–56, July 1983, 6309999, 10.1093/infdis/148.1.51,

Influenzavirus D

This genus has only one species, influenza D virus, which infects pigs and cattle. The virus has the potential to infect humans, although no such cases have been observed yet.

Structure, properties, and subtype nomenclature

Influenzaviruses A, B, C, and D are very similar in overall structure.International Committee on Taxonomy of Viruses descriptions of:weblink" title="">Orthomyxoviridae, weblink" title="">Influenzavirus B and weblink" title="">Influenzavirus CJOURNAL, Nakatsu S, Murakami S, Shindo K, Horimoto T, Sagara H, Noda T, Kawaoka Y, 6, Influenza C and D Viruses Package Eight Organized Ribonucleoprotein Complexes, Journal of Virology, 92, 6, e02084–17, March 2018, 29321324, 5827381, 10.1128/jvi.02084-17, The virus particle (also called the virion) is 80–120 nanometers in diameter such that the smallest virions adopt an elliptical shape.JOURNAL, Sugita Y, Noda T, Sagara H, Kawaoka Y, Ultracentrifugation deforms unfixed influenza A virions, The Journal of General Virology, 92, Pt 11, 2485–93, November 2011, 21795472, 3352361, 10.1099/vir.0.036715-0, The length of each particle varies considerably, owing to the fact that influenza is pleomorphic, and can be in excess of many tens of micrometers, producing filamentous virions.JOURNAL, Dadonaite B, Vijayakrishnan S, Fodor E, Bhella D, Hutchinson EC, Filamentous influenza viruses, The Journal of General Virology, 97, 8, 1755–64, August 2016, 27365089, 5935222, 10.1099/jgv.0.000535, However, despite these varied shapes, the viral particles of all influenza viruses are similar in composition. These are made of a viral envelope containing two main types of glycoproteins, wrapped around a central core. The central core contains the viral RNA genome and other viral proteins that package and protect this RNA. RNA tends to be single stranded but in special cases it is double.JOURNAL, Lamb RA, Choppin PW, The gene structure and replication of influenza virus, Annu. Rev. Biochem., 52, 467–506, 1983, 6351727, 10.1146/, Unusually for a virus, its genome is not a single piece of nucleic acid; instead, it contains seven or eight pieces of segmented negative-sense RNA, each piece of RNA containing either one or two genes, which code for a gene product (protein). For example, the influenza A genome contains 11 genes on eight pieces of RNA, encoding for 11 proteins: hemagglutinin (HA), neuraminidase (NA), nucleoprotein (NP), M1 (matrix 1 protein), M2, NS1 (non-structural protein 1), NS2 (other name is NEP, nuclear export protein), PA, PB1 (polymerase basic 1), PB1-F2 and PB2.JOURNAL, Ghedin E, Sengamalay NA, Shumway M, Zaborsky J, Feldblyum T, Subbu V, Spiro DJ, Sitz J, Koo H, Bolotov P, Dernovoy D, Tatusova T, Bao Y, St George K, Taylor J, Lipman DJ, Fraser CM, Taubenberger JK, Salzberg SL, 6, Large-scale sequencing of human influenza reveals the dynamic nature of viral genome evolution, Nature, 437, 7062, 1162–66, October 2005, 16208317, 10.1038/nature04239, 2005Natur.437.1162G, Hemagglutinin (HA) and neuraminidase (NA) are the two large glycoproteins on the outside of the viral particles. HA is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell, while NA is involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles.JOURNAL, Suzuki Y, Sialobiology of influenza: molecular mechanism of host range variation of influenza viruses, Biological & Pharmaceutical Bulletin, 28, 3, 399–408, March 2005, 15744059, 10.1248/bpb.28.399, Thus, these proteins are targets for antiviral drugs.JOURNAL, Wilson JC, von Itzstein M, Recent strategies in the search for new anti-influenza therapies, Current Drug Targets, 4, 5, 389–408, July 2003, 12816348, 10.2174/1389450033491019, Furthermore, they are antigens to which antibodies can be raised. Influenza A viruses are classified into subtypes based on antibody responses to HA and NA. These different types of HA and NA form the basis of the H and N distinctions in, for example, H5N1. There are 18 H and 11 N subtypes known, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.JOURNAL, Tong S, Zhu X, Li Y, Shi M, Zhang J, Bourgeois M, Yang H, Chen X, Recuenco S, Gomez J, Chen LM, Johnson A, Tao Y, Dreyfus C, Yu W, McBride R, Carney PJ, Gilbert AT, Chang J, Guo Z, Davis CT, Paulson JC, Stevens J, Rupprecht CE, Holmes EC, Wilson IA, Donis RO, 6, New world bats harbor diverse influenza A viruses, PLoS Pathogens, 9, 10, e1003657, 10 October 2013, 24130481, 3794996, 10.1371/journal.ppat.1003657, JOURNAL, Tong S, Li Y, Rivailler P, Conrardy C, Castillo DA, Chen LM, Recuenco S, Ellison JA, Davis CT, York IA, Turmelle AS, Moran D, Rogers S, Shi M, Tao Y, Weil MR, Tang K, Rowe LA, Sammons S, Xu X, Frace M, Lindblade KA, Cox NJ, Anderson LJ, Rupprecht CE, Donis RO, 6, A distinct lineage of influenza A virus from bats, Proceedings of the National Academy of Sciences of the United States of America, 109, 11, 4269–74, March 2012, 22371588, 3306675, 10.1073/pnas.1116200109, 2012PNAS..109.4269T,


(File:Virus Replication large.svg|thumb|Host cell invasion and replication by the influenza virus. The steps in this process are discussed in the text.)Viruses can replicate only in living cells.JOURNAL, Smith AE, Helenius A, How viruses enter animal cells, Science, 304, 5668, 237–42, April 2004, 15073366, 10.1126/science.1094823, 2004Sci...304..237S, Influenza infection and replication is a multi-step process: First, the virus has to bind to and enter the cell, then deliver its genome to a site where it can produce new copies of viral proteins and RNA, assemble these components into new viral particles, and, last, exit the host cell.JOURNAL, Bouvier NM, Peter Palese, Palese P, The biology of influenza viruses, Vaccine, 26 Suppl 4, D49–53, September 2008, 19230160, 10.1016/j.vaccine.2008.07.039, 3074182, Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and intestines of birds (Stage 1 in infection figure).JOURNAL, Wagner R, Matrosovich M, Klenk HD, Functional balance between haemagglutinin and neuraminidase in influenza virus infections, Reviews in Medical Virology, 12, 3, 159–66, May–June 2002, 11987141, 10.1002/rmv.352, After the hemagglutinin is cleaved by a protease, the cell imports the virus by endocytosis.JOURNAL, Steinhauer DA, Role of hemagglutinin cleavage for the pathogenicity of influenza virus, Virology, 258, 1, 1–20, May 1999, 10329563, 10.1006/viro.1999.9716, The intracellular details are still being elucidated. It is known that virions converge to the microtubule organizing center, interact with acidic endosomes and finally enter the target endosomes for genome release.Liu SL, Zhang ZL, Tian ZQ, Zhao HS, Liu H, Sun EZ, Xiao GF, Zhang W, Wang HZ, Pang DW (2011) Effectively and efficiently dissecting the infection of influenza virus by quantum dot-based single-particle tracking. ACS NanoOnce inside the cell, the acidic conditions in the endosome cause two events to happen: First, part of the hemagglutinin protein fuses the viral envelope with the vacuole's membrane, then the M2 ion channel allows protons to move through the viral envelope and acidify the core of the virus, which causes the core to disassemble and release the viral RNA and core proteins. The viral RNA (vRNA) molecules, accessory proteins and RNA-dependent RNA polymerase are then released into the cytoplasm (Stage 2).JOURNAL, Lakadamyali M, Rust MJ, Babcock HP, Zhuang X, Visualizing infection of individual influenza viruses, Proceedings of the National Academy of Sciences of the United States of America, 100, 16, 9280–85, August 2003, 12883000, 170909, 10.1073/pnas.0832269100, 2003PNAS..100.9280L, The M2 ion channel is blocked by amantadine drugs, preventing infection.JOURNAL, Pinto LH, Lamb RA, The M2 proton channels of influenza A and B viruses, The Journal of Biological Chemistry, 281, 14, 8997–9000, April 2006, 16407184, 10.1074/jbc.R500020200, These core proteins and vRNA form a complex that is transported into the cell nucleus, where the RNA-dependent RNA polymerase begins transcribing complementary positive-sense vRNA (Steps 3a and b).JOURNAL, Cros JF, Palese P, Trafficking of viral genomic RNA into and out of the nucleus: influenza, Thogoto and Borna disease viruses, Virus Research, 95, 1–2, 3–12, September 2003, 12921991, 10.1016/S0168-1702(03)00159-X, The vRNA either is exported into the cytoplasm and translated (step 4) or remains in the nucleus. Newly synthesized viral proteins are either secreted through the Golgi apparatus onto the cell surface (in the case of neuraminidase and hemagglutinin, step 5b) or transported back into the nucleus to bind vRNA and form new viral genome particles (step 5a). Other viral proteins have multiple actions in the host cell, including degrading cellular mRNA and using the released nucleotides for vRNA synthesis and also inhibiting translation of host-cell mRNAs.JOURNAL, Kash JC, Goodman AG, Korth MJ, Katze MG, Hijacking of the host-cell response and translational control during influenza virus infection, Virus Research, 119, 1, 111–20, July 2006, 16630668, 10.1016/j.virusres.2005.10.013, Negative-sense vRNAs that form the genomes of future viruses, RNA-dependent RNA polymerase, and other viral proteins are assembled into a virion. Hemagglutinin and neuraminidase molecules cluster into a bulge in the cell membrane. The vRNA and viral core proteins leave the nucleus and enter this membrane protrusion (step 6). The mature virus buds off from the cell in a sphere of host phospholipid membrane, acquiring hemagglutinin and neuraminidase with this membrane coat (step 7).JOURNAL, Nayak DP, Hui EK, Barman S, Assembly and budding of influenza virus, Virus Research, 106, 2, 147–65, December 2004, 15567494, 10.1016/j.virusres.2004.08.012, As before, the viruses adhere to the cell through hemagglutinin; the mature viruses detach once their neuraminidase has cleaved sialic acid residues from the host cell. After the release of new influenza viruses, the host cell dies.Because of the absence of RNA proofreading enzymes, the RNA-dependent RNA polymerase that copies the viral genome makes an error roughly every 10 thousand nucleotides, which is the approximate length of the influenza vRNA. Hence, the majority of newly manufactured influenza viruses are mutants; this causes antigenic drift, which is a slow change in the antigens on the viral surface over time.JOURNAL, Drake JW, Rates of spontaneous mutation among RNA viruses, Proceedings of the National Academy of Sciences of the United States of America, 90, 9, 4171–75, May 1993, 8387212, 46468, 10.1073/pnas.90.9.4171, 1993PNAS...90.4171D, The separation of the genome into eight separate segments of vRNA allows mixing or reassortment of vRNAs if more than one type of influenza virus infects a single cell. The resulting rapid change in viral genetics produces antigenic shifts, which are sudden changes from one antigen to another. These sudden large changes allow the virus to infect new host species and quickly overcome protective immunity. This is important in the emergence of pandemics, as discussed below in the section on epidemiology.



When an infected person sneezes or coughs more than half a million virus particles can be spread to those close by.BOOK, Sherman, Irwin W., vanc, Twelve diseases that changed our world, 2007, ASM Press, Washington, DC, 978-1-55581-466-3, 161, In otherwise healthy adults, influenza virus shedding (the time during which a person might be infectious to another person) increases sharply one-half to one day after infection, peaks on day 2 and persists for an average total duration of 5 days—but can persist as long as 9 days. In those who develop symptoms from experimental infection (only 67% of healthy experimentally infected individuals), symptoms and viral shedding show a similar pattern, but with viral shedding preceding illness by one day. Children are much more infectious than adults and shed virus from just before they develop symptoms until two weeks after infection.JOURNAL, Mitamura K, Sugaya N, [Diagnosis and Treatment of influenza—clinical investigation on viral shedding in children with influenza], Uirusu, 56, 1, 109–16, June 2006, 17038819, 10.2222/jsv.56.109, In immunocompromised people, viral shedding can continue for longer than two weeks.JOURNAL, Gooskens J, Jonges M, Claas EC, Meijer A, Kroes AC, Prolonged influenza virus infection during lymphocytopenia and frequent detection of drug-resistant viruses, The Journal of Infectious Diseases, 199, 10, 1435–41, May 2009, 19392620, 10.1086/598684, Influenza can be spread in three main ways:JOURNAL, Weber TP, Stilianakis NI, Inactivation of influenza A viruses in the environment and modes of transmission: a critical review, The Journal of Infection, 57, 5, 361–73, November 2008, 18848358, 10.1016/j.jinf.2008.08.013, JOURNAL, Hall CB, The spread of influenza and other respiratory viruses: complexities and conjectures, Clinical Infectious Diseases, 45, 3, 353–59, August 2007, 17599315, 10.1086/519433,weblinkweblink" title="">weblink dmy-all, live, 25 January 2016, by direct transmission (when an infected person sneezes mucus directly into the eyes, nose or mouth of another person); the airborne route (when someone inhales the aerosols produced by an infected person coughing, sneezing or spitting) and through hand-to-eye, hand-to-nose, or hand-to-mouth transmission, either from contaminated surfaces or from direct personal contact such as a handshake. The relative importance of these three modes of transmission is unclear, and they may all contribute to the spread of the virus. In the airborne route, the droplets that are small enough for people to inhale are 0.5 to 5{{nbsp}}µm in diameter and inhaling just one droplet might be enough to cause an infection. Although a single sneeze releases up to 40,000 droplets,JOURNAL, Cole EC, Cook CE, Characterization of infectious aerosols in health care facilities: an aid to effective engineering controls and preventive strategies, American Journal of Infection Control, 26, 4, 453–64, August 1998, 9721404, 10.1016/S0196-6553(98)70046-X, most of these droplets are quite large and will quickly settle out of the air. How long influenza survives in airborne droplets seems to be influenced by the levels of humidity and UV radiation, with low humidity and a lack of sunlight in winter aiding its survival.As the influenza virus can persist outside of the body, it can also be transmitted by contaminated surfaces such as banknotes,JOURNAL, Thomas Y, Vogel G, Wunderli W, Suter P, Witschi M, Koch D, Tapparel C, Kaiser L, 6, Survival of influenza virus on banknotes, Applied and Environmental Microbiology, 74, 10, 3002–07, May 2008, 18359825, 2394922, 10.1128/AEM.00076-08, doorknobs, light switches and other household items. The length of time the virus will persist on a surface varies, with the virus surviving for one to two days on hard, non-porous surfaces such as plastic or metal, for about fifteen minutes on dry paper tissues, and only five minutes on skin.JOURNAL, Bean B, Moore BM, Sterner B, Peterson LR, Gerding DN, Balfour HH, Survival of influenza viruses on environmental surfaces, The Journal of Infectious Diseases, 146, 1, 47–51, July 1982, 6282993, 10.1093/infdis/146.1.47, However, if the virus is present in mucus, this can protect it for longer periods (up to 17{{nbsp}}days on banknotes). Avian influenza viruses can survive indefinitely when frozen.WEB,weblink Influenza Factsheet, Center for Food Security and Public Health, Iowa State University, live,weblink 23 March 2009, p. 7 They are inactivated by heating to 56{{nbsp}}°C (133{{nbsp}}°F) for a minimum of 60 minutes, as well as by acids (at pH JOURNAL, Guerra F, The European-American exchange, History and Philosophy of the Life Sciences, 15, 3, 313–27, 1993, 7529930, The first convincing record of an influenza pandemic was of an outbreak in 1580, which began in Russia and spread to Europe via Africa. In Rome, over 8,000 people were killed, and several Spanish cities were almost wiped out. Pandemics continued sporadically throughout the 17th and 18th centuries, with the pandemic of 1830–1833 being particularly widespread; it infected approximately a quarter of the people exposed.The most famous and lethal outbreak was the 1918 flu pandemic (Spanish flu pandemic) (type A influenza, H1N1 subtype), which lasted from 1918 to 1919. It is not known exactly how many it killed, but estimates range from 50 to 100{{nbsp}}million people.BOOK, Knobler S, Mack A, Mahmoud A, Lemon S, The Threat of Pandemic Influenza: Are We Ready? Workshop Summary (2005), 1: The Story of Influenza, 60–61,weblink The National Academies Press, Washington, DC, JOURNAL, Patterson KD, Pyle GF, The geography and mortality of the 1918 influenza pandemic, Bulletin of the History of Medicine, 65, 1, 4–21, Spring 1991, 2021692, This pandemic has been described as "the greatest medical holocaust in history" and may have killed as many people as the Black Death. This huge death toll was caused by an extremely high infection rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms. Symptoms in 1918 were so unusual that initially influenza was misdiagnosed as dengue, cholera, or typhoid. One observer wrote, "One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and petechial hemorrhages in the skin also occurred." The majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive hemorrhages and edema in the lung.JOURNAL, Taubenberger JK, Reid AH, Janczewski TA, Fanning TG, Integrating historical, clinical and molecular genetic data in order to explain the origin and virulence of the 1918 Spanish influenza virus, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 356, 1416, 1829–39, December 2001, 11779381, 1088558, 10.1098/rstb.2001.1020, The 1918 flu pandemic was truly global, spreading even to the Arctic and remote Pacific islands. The unusually severe disease killed between two and twenty percent of those infected, as opposed to the more usual flu epidemic mortality rate of 0.1%. Another unusual feature of this pandemic was that it mostly killed young adults, with 99% of pandemic influenza deaths occurring in people under 65, and more than half in young adults 20 to 40 years old.JOURNAL, Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ, Fukuda K, Pandemic versus epidemic influenza mortality: a pattern of changing age distribution, The Journal of Infectious Diseases, 178, 1, 53–60, July 1998, 9652423, 10.1086/515616, This is unusual since influenza is normally most deadly to the very young (under age 2) and the very old (over age 70). The total mortality of the 1918–1919 pandemic is not known, but it is estimated that 2.5% to 5% of the world's population was killed. As many as 25{{nbsp}}million may have been killed in the first 25 weeks; in contrast, HIV/AIDS has killed 25{{nbsp}}million in its first 25 years.Later flu pandemics were not so devastating. They included the 1957 Asian Flu (type A, H2N2 strain) and the 1968 Hong Kong Flu (type A, H3N2 strain), but even these smaller outbreaks killed millions of people. In later pandemics antibiotics were available to control secondary infections and this may have helped reduce mortality compared to the Spanish flu of 1918.{|class="wikitable" style="text-align:center"flu pandemicsHILLEMAN MR, Realities and enigmas of human viral influenza: pathogenesis, epidemiology and control, Vaccine, 20, 25–26, 3068–87, 19 August 2002, 12163258, 10.1016/S0264-410X(02)00254-2, ! Name of pandemic !! Date !! Deaths !! Case fatality rate !! Subtype involved !! Pandemic Severity Index! 1889–1890 flu pandemic(Asiatic or Russian Flu)JOURNAL, Valleron AJ, Cori A, Valtat S, Meurisse S, Carrat F, Boëlle PY, Transmissibility and geographic spread of the 1889 influenza pandemic, Proc. Natl. Acad. Sci. USA, 107, 19, 8778–81, May 2010, 20421481, 10.1073/pnas.1000886107, 2889325, 2010PNAS..107.8778V, H3N8 or H2N2>|N/A! 1918 flu pandemic(Spanish flu)JOURNAL, Mills CE, Robins JM, Lipsitch M, Transmissibility of 1918 pandemic influenza, Nature, 432, 7019, 904–06, December 2004, 15602562, 10.1038/nature03063, 2004Natur.432..904M, H1N1 >|5! Asian FluH2N2 >|2! Hong Kong Flu|

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