Acomputervirusisacomputerprogramthatcancopyitselfandinfectacomputer.Theterm"virus"isalsocommonlybuterroneouslyusedtorefertoothertypesofmalware,includingbutnotlimitedtoadwareandspywareprogramsthatdonothavethereproductiveability.Atrueviruscanspreadfromonecomputertoanother(insomeformofexecutablecode)whenitshostistakentothetargetcomputer;forinstancebecauseausersentitoveranetworkortheInternet,orcarrieditonaremovablemediumsuchasafloppydisk,CD,DVD,orUSBdrive.Virusescanincreasetheirchancesofspreadingtoothercomputersbyinfectingfilesonanetworkfilesystemorafilesystemthatisaccessedbyanothercomputer.
Asstatedabove,theterm"computervirus"issometimesusedasacatch-allphrasetoincludealltypesofmalware,eventhosethatdonothavethereproductiveability.Malwareincludescomputerviruses,computerworms,Trojanhorses,mostrootkits,spyware,dishonestadwareandothermaliciousandunwantedsoftware,includingtrueviruses.VirusesaresometimesconfusedwithwormsandTrojanhorses,whicharetechnicallydifferent.Awormcanexploitsecurityvulnerabilitiestospreaditselfautomaticallytoothercomputersthroughnetworks,whileaTrojanhorseisaprogramthatappearsharmlessbuthidesmaliciousfunctions.WormsandTrojanhorses,likeviruses,mayharmacomputersystem'sdataorperformance.Somevirusesandothermalwarehavesymptomsnoticeabletothecomputeruser,butmanyaresurreptitiousorsimplydonothingtocallattentiontothemselves.Somevirusesdonothingbeyondreproducingthemselves.