influenza | Definition, Symptoms, Treatment, & Prevention

influenza, also called flu or grippe, an acute viral infection of the upper or lower respiratory tract that is marked by fever, chills, and a generalized ... influenza TableofContents influenza Introduction&TopQuestionsClassificationofinfluenzavirusesEvolutionandvirulenceofinfluenzavirusesPandemicsandepidemicsInfluenzapandemicpreparednessTransmissionandsymptomsTreatmentandprevention FastFacts 2-MinSummary Facts&RelatedContent Media Videos Images More MoreArticlesOnThisTopic Contributors ArticleHistory Home Health&Medicine Conditions&Diseases InfectiousDiseases influenza disease Actions Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. SelectCitationStyle MLA APA ChicagoManualofStyle CopyCitation Share Share Sharetosocialmedia Facebook Twitter URL https://www.britannica.com/science/influenza GiveFeedback ExternalWebsites Feedback Corrections?Updates?Omissions?Letusknowifyouhavesuggestionstoimprovethisarticle(requireslogin). FeedbackType Selectatype(Required) FactualCorrection Spelling/GrammarCorrection LinkCorrection AdditionalInformation Other YourFeedback SubmitFeedback Thankyouforyourfeedback Oureditorswillreviewwhatyou’vesubmittedanddeterminewhethertorevisethearticle. JoinBritannica'sPublishingPartnerProgramandourcommunityofexpertstogainaglobalaudienceforyourwork! ExternalWebsites WorldHealthOrganisation-Influenza CentersforDiseaseControlandPrevention-Flu TheNemoursFoundation-ForParents-Influenza TheNemoursFoundation-ForTeens-FluFacts NationalGeographic-Science-Influenza MayoClinic-Influenza WebMd-WhatistheFlu? BritannicaWebsites ArticlesfromBritannicaEncyclopediasforelementaryandhighschoolstudents. influenza-Children'sEncyclopedia(Ages8-11) influenza-StudentEncyclopedia(Ages11andup) Print print Print Pleaseselectwhichsectionsyouwouldliketoprint: TableOfContents Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. SelectCitationStyle MLA APA ChicagoManualofStyle CopyCitation Share Share Sharetosocialmedia Facebook Twitter URL https://www.britannica.com/science/influenza Feedback ExternalWebsites Feedback Corrections?Updates?Omissions?Letusknowifyouhavesuggestionstoimprovethisarticle(requireslogin). FeedbackType Selectatype(Required) FactualCorrection Spelling/GrammarCorrection LinkCorrection AdditionalInformation Other YourFeedback SubmitFeedback Thankyouforyourfeedback Oureditorswillreviewwhatyou’vesubmittedanddeterminewhethertorevisethearticle. JoinBritannica'sPublishingPartnerProgramandourcommunityofexpertstogainaglobalaudienceforyourwork! ExternalWebsites WorldHealthOrganisation-Influenza CentersforDiseaseControlandPrevention-Flu TheNemoursFoundation-ForParents-Influenza TheNemoursFoundation-ForTeens-FluFacts NationalGeographic-Science-Influenza MayoClinic-Influenza WebMd-WhatistheFlu? BritannicaWebsites ArticlesfromBritannicaEncyclopediasforelementaryandhighschoolstudents. influenza-Children'sEncyclopedia(Ages8-11) influenza-StudentEncyclopedia(Ages11andup) Alternatetitles:flu,grippe By TheEditorsofEncyclopaediaBritannica • EditHistory TableofContents influenzavirus Seeallmedia KeyPeople: ThomasFrancis,Jr. GiovanniMariaLancisi ...(Showmore) RelatedTopics: humanrespiratorysystem birdflu swineflu fowlplague universalinfluenzavaccine ...(Showmore) Seeallrelatedcontent→ TopQuestionsWhatisinfluenza?Influenzaisanacuteviralinfectionoftheupperorlowerrespiratorytractmarkedbyfever,chills,andageneralizedfeelingofweaknessandpaininthemusclestogetherwithvaryingdegreesofsorenessintheheadandabdomen.Whattypeofviruscausesinfluenza?InfluenzaiscausedbyseveralcloselyrelatedvirusesinthefamilyOrthomyxoviridae,anRNAvirusgroup.InfluenzavirusesarecategorizedastypesA,B,C,andD.Thesemajortypesgenerallyproducesimilarsymptomsbutareunrelatedantigenically.Theinfectionwithonetypeconfersnoimmunityagainsttheothers.Whatisthedifferencebetweeninfluenzaepidemicsandinfluenzapandemics?Pandemicsareworldwideoutbreaksofadiseaseandareestimatedtooccuronaverageonceevery50yearsforinfluenza.Epidemicshappenmuchmorefrequently,andseasonalinfluenzaappearsannuallyinmostpartsoftheworld.Theinfluenzapandemicof1918–19isthemostdestructiveinfluenzaoutbreakinhistory.Itresultedinanestimated25milliondeathsglobally.Whatispandemicinfluenzapreparedness?Whenaninfluenzapandemicemerges,WHOadherestoitssix-phasepreparednessplan.Phases1–3aretheearlystagesinpandemicpreparednessdesignedtopreventorcontainsmalloutbreaks.The4thand5thphasesarecharacterizedbyurgencyinimplementationofmitigationprocedures.Phase6ischaracterizedbywidespreaddiseaseandsustainedtransmissionofthevirusbetweenhumans.Whatarethesymptomsofinfluenza?Thesymptomsofinfluenzaincludesuddenanddistinctchills,fatigue,andmuscleaches.Aperson’sbodytemperaturerisesrapidlyto38–40°C(101–104°F).Therearediffuseheadachesandseveremuscularachesaccompaniedbyirritationorasenseofrawnessinthethroat.Symptomsassociatedwithrespiratorytractinfectionbecomemoreprominent,accompaniedbylingeringfeelingsofweakness.Summary Readabriefsummaryofthistopic influenza,alsocalledfluorgrippe,anacuteviralinfectionoftheupperorlowerrespiratorytractthatismarkedbyfever,chills,andageneralizedfeelingofweaknessandpaininthemuscles,togetherwithvaryingdegreesofsorenessintheheadandabdomen.ClassificationofinfluenzavirusesInfluenzaiscausedbyanyofseveralcloselyrelatedvirusesinthefamilyOrthomyxoviridae(agroupofRNAviruses).InfluenzavirusesarecategorizedastypesA,B,C,andD.Thesemajortypesgenerallyproducesimilarsymptomsbutarecompletelyunrelatedantigenically,sothatinfectionwithonetypeconfersnoimmunityagainsttheothers.TheAvirusescausethegreatinfluenzaepidemics,andtheBvirusescausesmallerlocalizedoutbreaks.TheCvirusescauseonlymildrespiratoryillnessinhumans.InfluenzaDvirusesarenotknowntoinfecthumansandhavebeenobservedonlyinpigsandcattle. BritannicaQuiz 44QuestionsfromBritannica’sMostPopularHealthandMedicineQuizzes Howmuchdoyouknowabouthumananatomy?Howaboutmedicalconditions?Thebrain?You’llneedtoknowalottoanswer44ofthehardestquestionsfromBritannica’smostpopularquizzesabouthealthandmedicine. InfluenzaAvirusesareclassifiedintosubtypes,andbothinfluenzaBandsubtypesofinfluenzaAarefurtherdividedintostrains.SubtypesofinfluenzaAaredifferentiatedmainlyonthebasisoftwosurfaceantigens(foreignproteins)—hemagglutinin(H)andneuraminidase(N).ExamplesofinfluenzaAsubtypesincludeH1N1,H5N1,andH3N2.InfluenzaBvirusesaresubdividedintotwomajorlineages,B/YamagataandB/Victoria.StrainsofinfluenzaBandstrainsofinfluenzaAsubtypesarefurtherdistinguishedbyvariationsingeneticsequence.EvolutionandvirulenceofinfluenzavirusesBetweenworldwideoutbreaks,knownaspandemics,influenzavirusesundergoconstant,rapidevolution(aprocesscalledantigenicdrift),whichisdrivenbymutationsinthegenesencodingantigenproteins.Periodically,thevirusesundergomajorevolutionarychangebyacquiringanewgenomesegmentfromanotherinfluenzavirus(antigenicshift),effectivelybecominganewsubtype.Viralevolutionisfacilitatedbyanimalssuchaspigsandbirds,whichserveasreservoirsofinfluenzaviruses.WhenapigissimultaneouslyinfectedwithdifferentinfluenzaAviruses,suchashuman,swine,andavianstrains,geneticreassortmentcanoccur.ThisprocessgivesrisetonewstrainsofinfluenzaA.Newlyemergedinfluenzavirusestendtobeinitiallyhighlyinfectiousandvirulentinhumansbecausetheypossessnovelantigenstowhichthehumanbodyhasnopreparedimmunedefense(i.e.,existingantibodies).Onceasignificantproportionofapopulationdevelopsimmunitythroughtheproductionofantibodiescapableofneutralizingthenewvirus,theinfectiousnessandvirulenceofthevirusdecreases.Althoughoutbreaksofinfluenzavirusesaregenerallymostfataltoyoungchildrenandtheelderly,thefatalityrateinpeoplebetweenages20and40issometimesunexpectedlyhigh,eventhoughthepatientsreceivetreatment.Thisphenomenonisbelievedtobeduetohyper-reactionoftheimmunesystemtonewstrainsofinfluenzavirus.Suchreactionresultsfromtheoverproductionofinflammatorysubstancescalledcytokines.Thereleaseofexcessiveamountsofthesemoleculescausessevereinflammation,particularlyintheepithelialcellsofthelungs.Individualswhoseimmunesystemsarenotfullydeveloped(suchasinfants)orareweakened(suchastheelderly)cannotgeneratesuchalethalimmuneresponse.PandemicsandepidemicsInfluenzapandemicsareestimatedtooccuronaverageonceevery50years.Epidemicshappenmuchmorefrequently,andseasonalinfluenzaappearsannuallyinmostpartsoftheworld,sometimesinepidemicproportions.InfluenzatypeAvirusisthemostfrequentcauseofseasonalinfluenza.WhenaninfluenzaAvirusundergoesanantigenicshift,apandemicaffectingmostoftheworldcanoccurwithinamatterofmonths.Theinfluenzapandemicof1918–19,themostdestructiveinfluenzaoutbreakinhistoryandoneofthemostseverediseasepandemicseverencountered,wascausedbyasubtypeofinfluenzaAknownasH1N1.Duringthispandemicanestimated25millionpersonsthroughouttheworlddiedoftheso-calledSpanishflu,whichwasfirstwidelyreportedinSpainbutoriginatedintheU.S.stateofKansas.influenzapandemicof1918–19:temporaryhospitalAtemporaryhospitalinCampFunston,Kansas,duringthe1918–19influenzapandemic.CourtesyoftheNationalMuseumofHealthandMedicine,ArmedForcesInstituteofPathology,Washington,D.CSubsequentpandemicsofinfluenzahavebeenlesssevere.Forexample,influenzaAsubtypeH2N2,or1957flupandemic,apparentlybeganinEastAsiaearlyin1957,andbymidyearithadcircledtheglobe.Theoutbreaklastedonapandemicleveluntilaboutthemiddleof1958andcausedanestimatedonemilliontotwomilliondeathsworldwide.After10yearsofevolutionthatproducedannualepidemics,the1957fludisappearedin1968,onlytobereplacedbyanewinfluenzaAsubtype,H3N2.Thisvirusisstillincirculation.Thefluoutbreakof1968wasthethirdinfluenzapandemicofthe20thcenturyandresultedinanestimatedonemilliontofourmilliondeaths.In1997atypeofavianinfluenza,orbirdflu,virusbrokeoutamongdomesticatedpoultryinHongKongandtheninfectedasmallnumberofpeople,killingsomeofthem.Thissamevirus,H5N1,reappearedamongchickenflocksinSoutheastAsiaduringthewinterof2003–04,againinfectingsomepeoplefatally,andithasreappearedperiodicallysince,primarilyinwildbirds,domesticpoultry,andhumans.Severalothersubtypesofbirdfluvirusesareknown,includingH7N2,H7N3,andH9N2.Thoughthesesubtypesrarelycauseinfectioninhumans,theyarerecognizedashavingepidemicandpandemicpotential.AnoutbreakofapreviouslyunknownstrainofH1N1occurredin2009.Originallycalledswineflubecausetheviruswassuspectedtohavebeentransmittedtohumansfrompigs,theillnessfirstbrokeoutinMexicoandthenspreadtotheUnitedStates.TheH1N1virusthatcausedtheoutbreakwasdiscoveredtopossessgeneticmaterialfromhuman,avian,andtwodifferentswineinfluenzaviruses.The2009H1N1outbreakwasnotnearlyasdeadlyasthepandemicof1918–19.However,theviruswashighlycontagiousandspreadrapidly.ThepandemicpotentialofthenewH1N1viruswasmadecleartotheinternationalcommunitybytheWorldHealthOrganization(WHO),whichdeclaredalevel5pandemicalertonApril29,2009.Thispromptedtherapidimplementationofmitigationprocedures,includingthedistributionofdrugstotreatmentfacilities,incountriesworldwide.Despitethesemeasures,theviruscontinuedtospreadglobally.OnJune11,2009,followinganincreaseincasesinChile,Australia,andtheUnitedKingdom,WHOraisedtheH1N1alertlevelfrom5to6,meaningthattheoutbreakwasofficiallydeclaredapandemic.Bymid-January2010theoutbreakhadaffectedpeopleinmorethan209countriesworldwide.Itwasthefirstinfluenzapandemicofthe21stcentury.IntheUnitedStatesthehighlevelsofflu-likeillnessobservedduringthe2009H1N1pandemicwerenotobservedagainuntil2018.ResearchhasindicatedthateachofthefourhistoricinfluenzapandemicswasprecededbyaLaNiñaevent—achangeinglobalweatherconditionsassociatedwithcoolseasurfacetemperaturesinthePacificOcean—which,somescientistsspeculate,mayhavealteredthemigratorypatternsofbirds,possiblyincreasingtheirinteractionswithdomesticanimalsandenablinggeneticreassortmentandtheriseofnewpandemicstrainsofinfluenzaviruses. NewfromBritannica Duringa1910meatshortage,theU.S.Congressconsideredabillfortheimportationofhipposforfarming. SeeAllGoodFacts