传染病英语教材

GENERALCONCEPTOFINFECTIOUSDISEASES

INTRODUCTION

Infectioncaninvolveanyorganorsystemofthebodyandthusembracesallmedical

disciplines.Inthischapterandintroductionisgiventothegeneralaspects,epidemiology,

diagnosis,preventionandtreatmentofinfectionaswellasdescriptionsofindividual

infectiousdiseases.

•Definitionofcommunicablediseases:Thegroupofdiseasescausedbyprion,virus,

chlamydia,richettisa,bacteria,fungus,spirochete,parasitewithcommunicabilityand

mayleadtoenepidemicunderproperconditions.

•Definitionofinfectiousdiseases:Thediseasescausedbyalllivingpathogens

includingcommunicablediseasesanduncommunicablediseases.

Thesubjectofinfectiousdiseasescloselyrelatedtomanysubjects,suchasmicrobiology,

immunology,parasitology,pathology,diagnostic,internalmedicineandsoon.Thegoalof

studyinginfectiousdiseasesislearningtheprincipleofdiseasehappeninganddevelopingin

body,transmission,clinicalmanifestations,diagnosis,differentdiagnosis,therapyand

preventionways.

infectiousdiseasesituation

Theworldisfullofmicroorganisms,thevastmajorityofwhichareharmlesstoman.

Infectiousdiseasescausenearly25%ofallhumandeaths(Table1).Infectionremainsthe

maincauseofmorbidityandmortalityinman,particularlyinunderdevelopedareaswhereit

isassociatedwithpovertyandovercrowding.

Manyofthedecimatingdiseasesofthepastarecontrollablebyvaccination(yellow

fever),vectorcontrol(malariaandsleepingsickness)andgeneralimprovementinliving

standards(plagueandrelapsingfever),butcontrolisimperfectandthediseasesreappear.

OtherepidemicdiseasessuchascholerainAsiaandmeningococcalmeningitisinAfrica

remainlargelyuncontrolled,andkillhundredsofthousandsofpeopleannually.Efficient

vaccinesexistformanydiseasessuchaspoliomyelitis,measles,rubella,meningitisand

tetanus,butinmanycountriestheyhavemadelittleimpactbecauseofcostandthepractical

difficultyindeliveringthem.

Inthe20thcenturytherehasbeenadramaticfallintheincidenceofcommunicable

diseasesindevelopedcountries.Thisisduetofactorssuchasimmunisation,antimicrobial

chemotherapy,improvednutrition,andbettersanitationandhousing.Infectiousdiseases

whichhavedecreased,andinsomeinstancesalmostdisappeared,includediphtheria,

poliomyelitisandtetanus.Smallpox,alethalvirusinfection,hasbeeneradicatedfromthe

worldwhileanotherlethalinfection,humanimmunodeficiencyvirus(HIV)infection,Sars,

Birdsflu.haveemerged.

Increasedglobalmobilityhasaidedthespreadofinfectiousdiseaseandallowed

previouslylocalizedpathogenstoestablishthemselvesworld-wide.Deterioratingsocial

conditionsintheinnercityareasofourmajorconurbationshavefacilitatedtheresurgenceof

tuberculosisandotherinfections.Changesinfarmingandfood-processingmethodshave

contributedtoanincreaseintheincidenceoffood-andwater-bornediseases.

Finally,indevelopingcountriesinfectiousdiseasesarefrequentlyassociatedwithnatural

disasterssuchasdrought,floodingandearthquakesaswellaswithwarandrevolution.

Table1World-widemortalityfrominfectiousdiseases

DiseaseEstimateddeaths(in1998)

Acutelowerrespiratoryinfection3.5million

HIV/AIDS2.25million

Diarrhealdisease2.25million

Tuberculosis1.5million

Malaria1.1million

Measles888000

Tetanus410000

Whoopingcough350000

Meningitis143000

Leishmaniasis42000

MICROORGANISMANDHOST

InteractionEffectsofinfectiononthebody

Eachofusiscolonizedwithhugenumbersofmicroorganisms(10TMbacteria,plus

viruses,fungi,protozoa,andworms)withwhichwecoexist.Therelationshipwithsomeof

theseorganismsissymbiotic,inwhichbothpartnersbenefit,whileothersarecommensals,

livingonthehostwithoutcausingharm.Infectionandillnessmaybeduetothesenormally

harmlesscommensalsandsymbiotesevadingthebod/sdefencesandpenetratinginto

abnormalsiteswhichiscalledopportunisticinfection.Alternatively,diseasemaybecaused

byexposuretoexogenouspathogenicorganismswhicharenotpartofournormalflora.

Definitionofinfectionisacourseofinteractionbetweenhostandorgamisms.Infection

hasmanyeffectsonthebody.Diseaseduetoinfectionistheresultofinteractionbetweena

microorganismandthedefencemechanismsofthebody.Theoutcomeofthisinteractioncan

rangefromnodemonstrableeffecttodeath,andwilldependonthequantity,mutationand

virulenceoftheorganisms,physiologicalandanatomicaleffectsthattheyinduce,andthe

effectivenessofthenaturaldefences.

Theoutcomeofinfection；

•Successfuleliminationofinvadingpathogensbynonspecificorspecificimmunity

(passiveoractive),infectioncannotoccur.

•Covert(sub-clinical)infection:Afterinvadingintobodypathogenevokeresponseof

specificimmunitywithoutorwithmildtissuedamage,solackofclinicalmanifestation.

Therearemaincharacters:usuallythemajority;specificimmunityacquired;carrier

state(asaresultofcovertinfection).

•Overt(clinical)infections:Afterinvadingintobodypathogennotonlyevoke

responseofspecificimmunitybutalsoleadtopathologicchangeandclinical

manifestatation.Therearemaincharacters:usuallyonlyaminority;specificstable

(measles,typhoid)orunstable(Flu.)immunityacquired;developeintocarriers.

•Carrierstate:apparentlyhealthypersonsexcretingpathogens,classificationof

carriers:

AClinicalclassification:

aIncubationperiodcarriers:excretionofpathogensduringincubationperiod.

bConvalescentcarriers:excretionofpathogensafterrecovery.

cHealthycarriers:excretionofpathogenswithoutanysymptomsandsigns.

BLaboratoryclassification:

aAcutecarriers:excretionofpathogensfbrlessthan3months;

bChroniccarriers:excretionofpathogensfbrmorethan3months;

•Latentinfection:Pathogensharbouredinsidethehostwithoutclinicalmanifestations

andnotexcretedtotheoutside.

Theroleplayedbypathogensintheinfectionprocess

Thepathogenicityofpathogens:

•Invasiveness:Directinvasion;invasionafterpreliminaryadhesion;invasionbymeansof

bacterialproducts,suchasflagellum.

•Virulence'.Toxins(exotoxinsandendotoxins)andtoxicproducts;penetrative,invasive

orcytolyticfactors.

•Infectiondose:Minimaldoseofinfectionaredifferentindifferentdiseases.

•Variability.Antigenicvariationofpathogenscanescapefromthehosts1immune

response.

Hostresponsetoinfection

Non-specificdefences

•Naturalbarriers'.extemal(skin,mucousmembrane),internal(blood-brain,placenta).

♦Phagocytosis:monocytes,macrophagesandgranulocyteseliminategranularpathogens.

•Humoralfactors'.complement,lysozyme,fibronectin,othercytokinesdirectlyor

indirectlyeliminatepathogensthroughimmunemodulation

Specificimmunity

Specificimmunityisacquiredbysomepathogeninfection.

■Cell-mediatedimmunity.Tcellsactivatedbyspecificantigencankillinfectedcellsand

theyalsosecreteavarietyofcytokineswhichattractandpromotetheantibacterialand

antiviralactivityofotherinflammatoryandimmunecells,especiallyintracelluarviral,

fungal,protozoanandcertainbacterialinfections.

•Humoralimmunity'.Bcellsactivatedbyspecificantigenproducedifferentkindsof

antibodieswhichcanneutralisebacterialtoxinsandbindtothesurfaceofmicroorganisms,

wheretheyinhibitspreadandinitiatecomplementfixationwhichpromotesphagocytosis

ofmicroorganisms.Thecomplementmembraneattackcomplexmaydestroycertain

bacteria,especiallyGram-negativeorganisms.

PRINCIPLESANDBASICMECHANISMS

Pathogenesis

Figure1summarizessomeofthestepsthatoccurduringthepathogenesisofinfection.

Specificity

•Someinfectiousagentsarestrictlyspecies-selective,forexample,Amoebiasisonly

naturallyaffectshumans.Evenwithinaspecies,relativeresistanceisapparent,suchas

thedecreasedsusceptibilityofDuffybloodgroup-negativeindividualstoPlasmodium

vivaxmalaria.

•Eachkindofpathogenshasitsfspecificportalofentry.Microorganismsarealso

highlyspecificwithrespecttotheorganortissuetheyinfect.Forexample,anumberof

virusesarehepatotropic,suchasthoseresponsiblefbrhepatitisA,B,CandEand

yellowfever.Thispredilectionforspecificsitesinthebodyrelatespartlytothe

immediateenvironmentinwhichtheorganismfindsitself;forexample,anaerobic

organismscolonizetheanaerobiccolon,whereasaerobicorganismsaregenerallyfound

inthemouth,pharynxandproximalintestinaltract.

•Withinanorganapathogenmayshowselectivityforaparticularcelltype.Inthe

intestine,fbrexample,rotaviruspredominantlyinvadesanddestroysintestinalepithelial

cellsontheupperportionofthevillus,whereasreovirusselectivelyentersthebody

throughthespecializedepithelialcells,knownasMcellsthatcoverthePeyer'spatches.

Epithelialattachment

Manybacteriaattachtotheepithelialsubstratumbyspecificorganellescalledpill(or

fimbriae)thatcontainasurfacelectin(s)-aproteinorglycoproteinthatrecognizesspecific

sugarresiduesonthehostcell.Thisfamilyofmoleculesisknownasadhesions.Some

viruses(e.g.HIV)andprotozoa(e.g.Plasmodium,Entamoebahistolytica)attachtospecific

target-cellreceptors.Otherparasitessuchashookwormhavespecificattachmentorgans

(buccalplates)thatfirmlygriptheintestinalepithelium.

Colonization

Followingepithelialattachment,pathogensmayeitherremainonthesurfaceepithelium

orwithinthelumenoftheorgantheyhavecolonized.Tissueinvasionmayfollow:

Invasionmayresultin:

•anintracellularlocationfbrthepathogen(e.g.viruses,Toxoplasmagondii,Leishmania

spp.,Plasmodiumspp.)

•anextracellularlocationfbrthepathogen(e.g.pneumococci,staphylococciand

Entamoebahistolytica)

•invasiondirectlyintothebloodorlymphcirculation(e.g.schistosomeschistosomula

andtrypanosomes)

Oncethepathogenisfirmlyestablishedinitstargettissue,aseriesofeventsfollowsthat

usuallyculminatesindamagetothehost.

Pathogenentry

Ep<thehalattachment

orinoculation

Muttiplicatioa

Colontzabon

Lum«na(/superf>cial

epithelralinfectoonTissuetnvasioa

e.g.V.cMerae

T.vaginalis

Dermatophytoses

IntracellularCirculation

(biood/lymph)

e.g.Virusese.g.Plasmodiumsp.

ToxoplasmaFiiariasis

IBacte”raemias

y

DysfunctionDestruction

e.g.Enterotoxine.g.Cytotoxin

(V.cholerae.E.coM(Diphtheria,

gasgangrene)

Fig.2.1Thepathogenesisofinfection.

Tissuedysfunctionordamage

Microorganismsproducediseasebyanumberofwelldefinedmechanisms:

•ExotoxinsandExotoxinshavemanydiverseactivitiesincludinginhibition

ofproteinsynthesis(diphtheriatoxin),neurotoxicity(ClostridiumtetaniandC.

botulinum)andenterotoxicity,whichresultsinintestinalsecretionofwaterand

electrolytes(E.coilVibriocholerae).Endotoxinisalipopolysaccharide(LPS)inthecell

wallofGram-negativebacteria.Itisresponsibleformanyofthefeaturesofsepticshock,

namelyhypotension,fever,intravascularcoagulationand,deathathighdoses.The

effectsofendotoxinaremediatedpredominantlybyreleaseoftumournecrosisfactor.

•Tumournecrosisfactor(TA^F)zTNF-alphaisreleasedfromavarietyofphagocytic

cells(macrophages/monocytes)andTNF-betafromnon-phagocyticcells(lymphocytes,

naturalkillercells)inresponsetoinfectionsandinflammatorystimuli.TNFitselfthen

stimulatesthereleaseofacascadeofothermediatorsinvolvedininflammationand

tissueremodelling,suchasinterleukin(IL-1andIL-6),prostaglandins,leukotrienesand

corticotropin.TNFisthereforeresponsiblefbrmanyoftheeffectsofaninfection.

•Tissueinvasion:Staphylococcusaureushastissue-invasivequalitiesallowingabscess

formationandbacteraemia,aswellasproducingtoxinswhichcancausediarrhoeaand

widespreaderythema(staphylococcalscaldedskinsyndrome).Similarly,some

pathogenicE.colicanproducetissueinvasionwithoutproductionofaspecifictoxin.

•Secondaryimmunologicalphenomena:organismscaninitiatesecondary

immunologicalmechanisms,suchascomplementactivation,immunecomplex

formationandantibody-mediatedcytolysisofcells,whichmayleadtotissuedamage.

Manyinfectionsareself-limiting,andimmuneandnon-immunehostdefence

mechanismswilleventuallyclearthepathogens.Thisisgenerallyfollowedbytissue

repair,whichmayresultincompleteresolutionorleaveresidualdamage.

Metabolicandimmunologicalconsequencesofinfection

Fever

Bodytemperatureiscontrolledbythethermoregulatorycentreintheanterior

hypothalamusinthefloorofthethirdventricle.IL-1,IL-6andTNF-aarereleasedfroma

varietyofcellsinvolvedinhostdefence,primarilybloodmonocytesandphagocytes,under

theinfluenceofmicrobialexogenouspyrogenssuchaslipopolysaccharide(LPS).These

cytokinesactonthethermoregulatorycentrebyincreasingprostaglandin(PGE2)synthesis.

Theantipyreticeffectofsalicylatesisbroughtabout,atleastinpart,throughitsinhibitory

effectsonprostaglandinsynthetase.

Feverproductionhasapositiveeffectonthecourseofinfection.However,forevery

loCriseintemperature,thereisa13%increaseinrestingmetabolicrateandoxygen

consumption.Feverthereforeleadstoincreasedenergyrequirementsatatimewhen

anorexialeadstodecreasedfoodintake.Thenormalcompensatorymechanismsinstarvation

(e.g.mobilizationoffatstores)areinhibitedinacuteinfections.Thisleadstoanincreasein

skeletalmusclebreakdown,releasingaminoacids,which,viagluconeogenesis,areusedto

provideenergy.

Inchronicinfectionthereistimeforadaptation.Thebodyisabletoutilizefatstores

moreeffectively,andthusweightlossismuchslower.

Proteinmetabolism

Duringacuteinfection,threemajorchangesoccurinproteinmetabolism:

•Thereisadiversionofsynthesisawayfromsomaticandcirculatingproteinssuchas

albumintowardsacute-phaseproteins.

•Proteinsynthesisisalsodirectedtowardsimmunoglobulinproductionandthereis

increasedproductionoflymphocytes,neutrophilsandotherphagocyticcells.

•Thereisamarkedincreaseinnitrogenlossesduetotissuebreakdown,whichmayreach

10-15gramperday.

Nutritionandhostdefence

Undemutritionimpairshostdefence.Naturalresistancetoinfectionisloweredby

alterationsintheintegrityofbodysurfaces,thereducedabilitytorepairepithelia,andthe

reductioningastricacidproduction.Inaddition,withmalnutrition,immunological

abnonnalitiesarefound.

Mineralmetabolismandacid-basebalance

Mineralmetabolismandacid-basebalancearedisturbedduringacuteinfection.In

general,sodiumandwaterareretained,principallyowingtotheeffectsofincreasedlevelsof

aldosteroneandinappropriatesecretionofantidiuretichormone.Duringtheconvalescent

periodafteracuteinfection,adiuresismayoccur.Acid-basebalancedisturbanceiscommon.

Causesincluderespiratoryalkalosisfollowingtachypnoearelatedtofever,respiratory

acidosisandhypoxaemiaassociatedwithpneumonia,andmetabolicacidosisassociatedwith

septicaemia.

Inacuteinfectionthesechangesaremildandresolvepromptlywithoutspecific

intervention.However,insituationswhereinfectionsareprolongedandresolutionisslow,

supportivecaremaybenecessary,particularlywithrespecttomanagingnutritionaldeficits

andelectrolyteandacid-basedisturbances.

THEEPIDEMIOLOGYPROCESSOFINFECTIOUSDISEASE

ANDINFLUENCING

Epidemiologyprocessistheprocessofoccurrence,developmentandoutcomeofinfectious

diseases.

Essentialelementsofepidemiologicprocess

Sourcesofinfection

Definitionsourcesofinfection:Humanbeingsandanimalsinwhichpathogenscansurvive,

multiplyand,excretedtotheenvironment.

•Theendogenouscommensals:skinandbowelcommensalscancausediseaseinthe

host,eitherbecausetheyhavebeentransferredtoaninappropriatesite(e.g.bowel

colifbrmscausingurinarytractinfection),orbecausehostimmunityhasbeenattenuated

(e.g.candidiasisinanimmunocompromisedhost).

•Theexogenoussource:Manyinfectionsareacquiredfromotherpeople.

Patients:Acute,chronic,typical,atypical.

Subclinicalinfections(poliomyelitis,viralhepatitis).

Carriers:Acute,chronic,convalescent,healthy(typhoidfever).

•Zoonoses:Infectionscanbetransmittedfromwildordomesticinfectedanimalstoman.

Infectioncanbeacquiredinanumberofways:directcontactwiththeanimal,ingestion

ofmeatoranimalproducts,contactwithanimalurineorfaeces,aerosolinhalation,via

anarthropodvector,orbyinoculationofsalivainabitewound.Manyzoonosescanalso

betransmittedfrompersontoperson.

•EnvironmentalsourcesMostmicroorganismsdonothaveavertebrateorarthropodhost

butarefree-livingintheenvironment.Thevastmajorityoftheseenvironmental

organismsarenon-pathogenic,butafewcancausehumandisease.Person-to-person

transmissionoftheseinfectionsisrare.Someparasitesmayhaveastageoftheirlife

cyclewhichisenvironmental(fbrexamplethefree-livinglarvalstageofStrongyloides

stercoralisandthehookworms)eventhoughtheadultwormrequiresavertebratehost.

Otherpathogenscansurvivefbrperiodsinwaterorsoilandmaybetransmittedfrom

hosttohostviathisroute(seebelow):theseshouldnotbeconfusedwithtrue

environmentalorganisms.

Routesoftransmission

•Endogenousinfection

Thebody'sownendogenousfloracancauseinfectioniftheorganismgainsaccesstoan

inappropriateareaofthebody.Thiscanhappenbysimplemechanicaltransfer,forexample

colonicbacteriaenteringthefemaleurinarytract.Thenon-specifichostdefencesmaybe

breached,fbrexamplebycuttingorscratchingtheskinandallowingsurfacecommensalsto

gainaccesstodeepertissues;Theremaybemoreseriousdefectsinhostimmunityowingto

diseaseorchemotherapy,allowingnormallyharmlessskinandbowelfloratoproduce

invasivedisease.

•Airbornespread

Manyrespiratorytractpathogensarespreadfrompersontopersonbyaerosolordroplet

transmission.Secretionscontainingtheinfectiousagentarecoughed,sneezed,orbreathed

out,andaretheninhaledbyanewvictim.Someentericviralinfectionsmayalsobespread

byaerosolsoffaecesorvomit.Someenvironmentalandzoonosespathogensarealso

acquiredbyaerosolinhalation,.

•Faeco-oralspread

AFood-bornefaeco-oral:Humanandanimalfaecalpathogenscangetintothefood

supplyatanystage.Rawsewageisusedasfertilizerinmanypartsoftheworld,

contaminatinggrowingvegetablesandfruit.Poorpersonalhygienecanresultin

contaminationduringproduction,packaging,preparationorservingoffoodstuffs.In

thewesternworld,thecentralizationoffoodsupplyandincreasedprocessingof

foodhasallowedthepotentialforrelativelyminorepisodesofcontaminationto

causewidelydisseminatedoutbreaksoffbod-bomeinfection.

BWater-bornefaeco-oralspreadisusuallytheresultofinadequateaccesstoclean

waterandsafesewagedisposal,andiscommonthroughoutthedevelopingworld.In

wealthiercountriesitusuallyoccursinisolatedoutbreakswithaspecificsource.

•Contagium

Respiratoryanddigesttractinfectiousdiseasecanbetransmittedbydirecttransfer

(usuallyinsmallchildren),bycontaminationofclothingorhouseholditems(usuallyin

institutionsorconditionsofpoorhygiene).Someparasitediseaseisinfectedbysoilsuchas

hookworm,Ascariasis.

・Vector-bornedisease

Manytropicalinfections,mostimportantlymalaria,arespreadfrompersontopersonor

fromanimaltopersonbyanarthropodvector.Vector-bornediseasesarealsofoundin

temperateclimates,butarerelativelyuncommon.Inmostcasespartoftheparasitelifecycle

takesplacewithinthebodyofthearthropod,andeachparasitespeciesrequiresaspecific

vector.Simplemechanicaltransferofinfectiveorganismsfromonehosttoanothercanoccur,

butisrare.Somevector-bornediseasesareshowninTable2

Table2Infectionstransmittedbyarthropodvectors

DiseaseInfectiveorganismVector

DengueFlavivirusMosquito

YellowfeverFlavivirusMosquito

ScrubtyphusRickettsiatsutsugamushiMite

RickettsialspottedfeversRickettsiasp.Hardtick

Tick-bornerelapsingfeverBorreliaduttoniSofttick

Louse-bornerelapsingfeverBorreliarecurrentisBodylouse

LymediseaseBorreliaburgdorferiHardtick

PlagueYersiniapestisFlea

MalariaPlasmodiumsp.Mosquito

LymphaticfilariasisWuchereriabancroftiMosquito

BrugiamalayiMosquito

OnchocerciasisOnchocercavolvulusBlackfly

LeishmaniasisLeishmaniasp.Sandfly

AfricantrypanosomiasisTrypanosomabruceiTsetsefly

SouthAmericantrypanosomiasisTrypanosomacruziReduviidbug

•Directperson-to-personspread

ASexually-transmittedinfections,

BSkin-to-skincontacting;

CBlood-(oroccasionallyotherbodyfluid)to-bloodtransmission:insomecasessuch

asHIVandhepatitisBvirusthisistheonlyrouteinotherssuchasmalariaitisan

unusualalternativetothenormalarthropodvector;contaminatedbloodorblood

productsaretransfused

DMothertoinfant;

EInjecting:betweendruguserssharinganypartoftheirinjectingequipment;infected

medicalequipmentisreused;

FSportingoraccidentalcontact:whenbloodisspilled.

Consumptionofinfectedmaterial

Severalimportantdiseasesaretransmitteddirectlyinanimalproducts.Theseinclude

somestrainsofsalmonella(eggs,chickenmeat),brucellosis(unpasteurizedmilk),andthe

priondiseaseskuruandvCJD(neuraltissue).

Susceptiblepopulation

Thesusceptibilityofpopulationdependontheproportionofsusceptibleindividualwhois

lackofspecificimmuneinthepopulation.Whentheproportionreachsomescaleandthere

arepropersourceofinfection,theroutesoftransmissionthenthediseasewillspread.

Factorsinfluencingtheepidemiologicalprocess.

Naturalfactors

Therearecloserelationshipbetweennaturefactorsandareaandseasonincluding

climaticconditions(seasonality),geographicconditions,(endemicity),biologicenviroment.

Thenaturefactorsplayanimportantroleinparasitosisandvector-bomediseases,for

example,malariaisepidemicintropiczone.

Socialfactors

Socialsystems,socioeconomicconditions,hygienecondition,andculturalbackgrounds

greatlyinfluencetheepidemicofinfectiousdisease.

CHARACTERISTICSOFCOMMUNICABLEDISEASE

BasicCharacteristics

Communicablediseasesdifferfromotherdiseasesinanumberofaspects.

Causedbylivingpathogen

Pathogencanusuallybeidentified,thusestablishing,theaetiologyearlyintheillness.

Manyoftheseorganism,includingallbacteria,aresensitivetoantibioticsandmost

infectionsarepotentiallycurable,unlikemanynon-infectiousdiseaseswhichare

degenerativeandfrequentlybecomechronic.

Communicability

Communicablilityisanotherfactorwhichdifferentiatesinfectionsfrom

non-communicablediseases.SuchasSarsisdifferentfromtraditionalatypicpneumonia

Communicabilitymeanspathogencanpassonformonetoanotherone.Periodofinfectivity

isthetimeduringwhichthepatientisinfectioustoothers.Differentkindofcommunicable

diseasehasdifferentinfectivityperiod.

Epidemiologicfeatures

Transmissionofpathogenicorganismstootherpeople,directlyorindirectly,maylead

toanepidemicfinally.Theepidemicofdiseasecanhappenthroughimport(cholera)or

originfromlocalareaorhome.

Generalfeatures:incidenceofadiseaseinage,sex,seasonandgeographicdistribution.

Incidence:casesinper100,000populationperyear.Accordingtoepidemicscalethereare4

typeofepidemic:

•Sporadicoccurrence,.theincidenceofcommunicablediseasesisaroundthemedian

incidenceinpast5years.

•Epidemicstheincidenceofcommunicablediseasesissignificantlyhigherthanthe

medianofincidenceofthediseaseinpast5years.

•Epidemicoutbreak*Alargenumberofcasesoccurwithinashortperiodbyasame

sourceofinfectionandasamerouteoftransmission.

•Pandemic:Epidemicofcommunicablediseasesspreadsacrosstheborderofacountry

oracontinent.

Post-infectionimmunity

Specificimmunityacquireaftercovertinfectionorovertinfectionwhichcanbe

detectedbyimmunoassay.Theimmunitylastingvaryindifferentdisease,usuallyoccurs

afterrecoveryofcomunicabtediseases.

ClinicalCharacteristics

Differentstagesoftheclinicalcourses

•Incubationperiod.Incubationperiodistheperiodbetweentheinvasionofpathogens

andtheonsetofdisease.Itisusefulfordiagnosisanddecidinghowlong