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Thecomputervirusisanoutcomeofthecomputerovergrowthinthe1980s．Thecauseofthetermcomputervirusisthelikenessbetweenthebiologicalvirusandtheevilprograminfectedwithcomputers．TheoriginofthistermcamefromanAmericansciencefiction“TheAdolescenceofP－1“writtenbyThomasJ.Ryan，publishedin1977．Humanvirusesinvadealivingcellandturnitintoafactoryformanufacturingviruses.However,computervirusesaresmallprograms.Theyreplicatebyattachingacopyofthemselvestoanotherprogram.OnceattachedtomehostProgram，thevirusesthenlookforotherprogramstoinfect．Inthisway，heviruscanspreadquicklythroughoutaharddiskoranentireorganizationwhenitinfectsaLANoramulti-usersystem．Atsomepoint，determinedbyhowtheviruswasprogrammedthevirusattacks．Thetimingoftheattackcanbelinkedtoanumberofsituations，includingacertaintimeordate，thepresenceofaparticularfile，thesecurityprivilegeleveloftheuser，andthenumberoftimesafileisused．Likewise，themodeofattackvaries．So－calledbenignvirusesmightsimplydisplayamessage，liketheonethatinfectedIBM’smaincomputersystemlastChristmaswithaseasonsgreeting．Malignantvirusesaredesignedtodamagethesystem．Theattackistowipeoutdata，todeleteflies，ortoformattheharddisk.WhatKindofVirusesAreThere？Therearefourmaintypesofviruses:shell,intrusive,operatingsystemandsourceode．Shellviruseswrapthemselvesaroundahostprogramanddontmodifytheoriginalprogram．Shellprogramsareeasytowrite，whichiswhyabouthalfofvirusesareofthistype．IntrusivevirusesInvadeanexistingprogramandactuallyinsertaportionofthemselvesintothehostprogram．Intrusivevirusesarehardtowriteadverydifficulttoremovewithoutdamagingthehostfile.Shellandintrusivevirusesmostcommonlyattackexecutableprogramflies－thosewitha．cornor．exeextension，althoughdatafliesarealsoatsomerisk．Operatingsystemvirusesworkbyreplacingpartsoftheoperatingsystemwiththeirownlogic．Itisverydifficulttowriteoperatingsystemvirusesandtheseviruseshavetheabilityoncebootedup，totaketotalcontrolofyourSystem．FOFexample，someoperatingsystemviruseshavehiddenlargeamountsofattacklogicinfalselymarkedbaddisksectors．SourcecodevirusesareintrusiveprogramsandtheyarealsoinsertedintoasourceprogramsuchasthosewritteninPascalpriortotheprogrambeingcomplied．Therearetheleast－commonvirusesbecausetheyarenotonlyhardtowrite，butalsohavealimitednumberofhostscomparedtotheothertypes．BeWaryoftheSecondNetworkVirusesDoyoubelieveit？Networkvirusescanstealmoney！SofarInternethasbecomethemainchannelthroughwhichthecomputervirusesspread．Look，herecomethesecondnetworkcomputerviruses．Evenwithoutsnatchinginformationfromthenetworkyourcomputercanbeinfectedbythesecondnetworkcomputerviruses，whicharehiddeninsomemachinesonthenetwork．Yourcomputeris，sotospeak，indangeroncebeingconnectedtothenetworktobrowse．Thevirusthatcanstealyourmoneybelongstoakindofthesecondnetworkviruses.Itwasdesignedandputinsomemachinesonthenetwork．Whenyourcomputerislinkedtooneofthesemachines，theviruswillinvadeyourharddiskandsearchwhetherIntuitQuicken，anaccountingsoftware，isinstalled．Oneofthisaccountingsoftwaresfunctionsistotransferaccountsautomatically.Onceinfected，yourmoneywillbetransferredtoanadditionalaccountopenedbythevirusprogramwithoutanybodyknowingt．