外壳型病毒常附着在主程序的首尾，在文件执行时先行执行此病毒程序，从而不断的复制，使计算机工作效率降低，最终使计算机死机.
外壳型病毒将它们自己包裹在主程序的四周，对源程序不作修改。 外壳型病毒容易编写，这也正是约有一半病毒程序是这种类型的原因。
Shell viruses wrap themselves around a host program and don't modify the original program．Shell programs are easy to write，which is why about half of viruses are of this type．
病毒是只有少数基因和蛋白外壳的紧凑型生物实体，对生物工程有很大作用。
Compact biological entities with little more than a handful of genes and a protein coat, viruses lend themselves to engineering.
有四种类型的病毒程序：外壳型、入侵型、操作系统型和源码型。
There are four main types of viruses: shell, intrusive, operating system and source ode.
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