四级快速阅读训练.docx
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四级快速阅读训练.docx
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四级快速阅读训练
第一篇
Directions:
Inthispart,youwillhave15minutestogooverthepassagequicklyandanswerthequestionsonAnswerSheet1.Forquestions1-7,choosethebestanswerfromthefourchoicesmarked[A],[B],[C]and[D].Forquestion8-10,completethesentenceswiththeinformationgiveninthepassage。
HowInfectiousDiseasesWork
Thehumanbodyisbothsurroundedandinhabitedbybillionsofmicroorganisms.Mostmicroorganismsareharmlessorevenbeneficial;forexample,bacteriathatnormallyliveinthedigestivesystemhelpdigestfood.Occasionally,however,amicroorganismcapableofcausingadiseaseinvadesthebody.Diseasescausedbysuchmicroorganismsarecalledinfectiousdiseases.
Infectiousdiseasesarecontagious;thatis,theycanbepassedfromonepersontoanother.Theycanbetransmittedbyskincontact,throughbodyfluids,incontaminatedfoodordrink,orviaairborneparticlescontainingthemicroorganisms,althoughthepathwaysandeaseoftransmissionvarybydisease.
Animalorinsectbitesareanothermeansoftransmission.Thetwomostcommontypesofinfectiousdiseasesarebacterialinfectionsandviralinfections.
Disease-causing,orpathogenic,bacteriaeitherattackthebody’stissuesdirectlyorcausedamagebysecretingpoisonoussubstancescalledtoxins.Fortunately,bacterialinfectionsareoftencurable.Certainbacteriacanbekilledbydrugs;otherbacterialdiseasescanbepreventedbyvaccination.
Virusesarethesmallestknownmicroorganisms.Theyareresponsiblefordiseasesasrelativelyharmlessasthecommoncoldandasseriousasmeningitis.Virusesliveandreproduceonlywithinlivingcells,andonlycertaincellsaresusceptibletoaspecificvirus.Youcanbehosttomanyviruseswithoutsufferinganyadverseeffects,butifenoughcellsareattacked,youwillbecomesick.
Thereisnoeffectivemedicaltreatmentformostviralinfections.Becauseaviruslivesinsideacell,anytreatmentdesignedtokillthevirusisalsolikelytoharmthecell.Inaddition,therearethousandsofdifferentviruses—eachonewithdifferentproperties—andanagenteffectiveagainstonevirusprobablywillnotaffecttheothers.Althoughtherearevaccinationstoprotectagainstsomeviraldiseases,therapyformostviraldiseasesislimitedtotreatingthesymptoms.
Inthisarticle,we’llfocusonthemanyfacetsofinfectiousdiseases,startingwithhowthebodydefendsagainstthem.
TheBody’sDefenses
Despitetheprevalenceofdisease-causingmicroorganisms,thebodyisnotdefenselessagainsttheseinvaders.Thebodyfightsinfectionsinthreeways:
bypreventingtheorganismsfromenteringthebody,byattackingthosethatdomanagetoenter,andbyinactivatingthoseorganismsitcannotkill.
Sometimes,too,thebodyfightsdiseasebydevelopingdefensivesymptoms.Feverisanexample.Duringanillness,thebody’stemperatureregulatormayrespondtotheillnessbyraisingthebody’stemperature.Someresearchersbelievethatthisisaneffectiveresponsebecausethemicroorganismscausingthediseasemaynotbeabletosurvivethehigherbodytemperature.
Theskinisthefirstbarrierthatguardstheunderlyingtissuesofthebody.Wheretherearenaturalopeningsintheskin,therearealsodefenses.Forexample,tearglandsintheeyessecreteandbathetheeyeswithfluidthatcontainsbacteria-fightingcomponents.Thesalivaryglandsinthemouthandthetonsilsinthethroathelppreventmicroorganismsfromattackingthemouthandthroat.
Manyopenings,aswellasinternalpassages,inthebodyarelinedwithmucousmembranes.Thesedelicatelayersproducemucus,aslipperysecretionthatmoistensandprotectsbyrepellingortrappingmicroorganisms.
Internally,certainbodyorgansfightinfection.Forinstance,theliverandthespleen(alargeglandlikeorganlocatedintheabdomen)filteroutharmfulsubstancesfromthebloodflowingthroughthem.Theliningofthestomachproducesacidsthatattackgermsinfoodthathasbeeneaten.Thebody’slymphsystemmanufactureswhitebloodcells,whichattackandkillinvadingorganisms.
Nowlet’sgetevenmorespecificinourlookatthebody’sdefenses.We’llstartbydescribingthelymphsystem.
TheLymphSystem
Thelymphsystemisanetworkofvesselsthatcarrylymph,awateryfluidcontainingwhitebloodcells,throughoutthebody.Lymphdrainsfromthebloodvesselsandbodytissues,carryingawaywasteproducts.Thewasteproductsarefilteredoutofthelymphbysmallstructurescalledlymphnodes.Withinthelymphnodes,harmfulmicroorganismsaretrapped,attacked,anddestroyedbywhitebloodcells.Thisisoneofthebody’sprimaryandmostefficientlinesofdefense.
Antibodiesaremanufacturedinthelymphsystem.Antibodiesareprotectivesubstancesthatthebodyproducesinresponsetoinvasionbyahostileorganismorthepresen
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