Network　immunization　strategies　have　emerged　as　possible　solutions　to　the　challenges　of　virus　propagation.　In　this　paper,　an　existing　interactive　model　is　introduced　and　then　improved　in　order　to　better　characterize　the　way　a　virus　spreads　in　email　networks　with　different　topologies.　The　model　is　used　to　demonstrate　the　effects　of　a　number　of　key　factors,　notably　nodes＇　degree　and　betweenness.　Experiments　are　then　performed　to　examine　how　the　structure　of　a　network　and　human　dynamics　affects　virus　propagation.　The　experimental　results　have　revealed　that　a　virus　spreads　in　two　distinct　phases　and　shown　that　the　most　efficient　immunization　strategy　is　the　node-betweenness　strategy.　Moreover,　those　results　have　also　explained　why　old　virus　can　survive　in　networks　nowadays　from　the　aspects　of　human　dynamics.