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aidsistheabbreviation1)ofacquiredimmunedeficiencysyndrome．itisacomplicatedillnessthatmayinvolveseveralphases．itiscausedbyavirusthatcanbepassedfrompersontoperson．aidsimpairsthehumanbody'simmunesystem，thesystemresponsibleforkeepingoffdisease，andleavesthevictimeasilyaffectedbyvariousinfection．
thevirusentersthebloodstreamanddestroyscertainwhitebloodcells，calledtlymphocytes2)，whichplayakeyroleinthefunctioningoftheimmunesystem．theviruscanalsoinfectothertypesofcellsinthebody，includingtheimmune－systemcellsknowsasmacrophages3)．unliketlymphocytes，however，macrophagesarenotkilledbythevirus．
mostpeoplerecentlyinfectedbytheaidsviruslookandfeelhealthy．insomepeoplethevirusmayremaininactive，andthesepeopleactascarriers，remainingapparentlyhealthybutstillabletoinfectothers．afterafewyears，somepeoplemaydevelopaids－relatedcomplex，orarc．itssymptomsmayincludefever，fatigue4)，weightloss，skinrashes，afungal5)infectionofthemouthknownasthrush，lackofresistancetoinfection，andswollenlymphnodes．
aidsisspreadbydirectinfectionofthebloodstreamwithbodyfluidsthatcontaintheaidsvirus，particularbloodandsemenfromanhiv－infectedperson．thevirusisusuallyspreadthroughvariousformsofsexualintercourse，thetransfusionofvirus－infectedblood，orthesharingofhiv－contaminatedintravenous6)needles．