Intrusive viruses invade an existing program and actually insert a portion of themselves into the host program. Intrusive viruses are hard to write and very difficult to remove without damaging the host file.

• 入侵型病毒侵入已有的程序，实际上是把病毒程序的一部分插入到主程序中。入侵型病毒难以编写，在去除它们时常常会破坏主文件。