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The computer virus is an outcome of the computer overgrowth in the 1980s. The cause of the term computer virus is the likeness between the biological virus and the evil program infected with computers. The origin of this term came from an American science fiction The Adolescence of P-1written by Thomas J. Ryan,published in 1977. Human viruses invade a living cell and turn it into a factory for manufacturing viruses. However,computer viruses are small programs. They replicate by attaching a copy of themselves to another program. Once attached to the host Program,the viruses then look for other programs to infect.In this way, the virus can spread quickly throughout a hard disk or an entire organization when it infects a LAN or a multi-user system. At some point,determined by how the virus was programmed the virus attacks. The timing of the attack can be linked to a number of situations,including a certain time or date, the presence of a particular file,the security privilege level of the user,and the number of times a file is used. Likewise,the mode of attack varies. So-called benign viruses might simply display a message,like the one that infected IBMs main computer system last Christmas with a seasons greeting. Malignant viruses are designed to damage the system. The attack is to wipe out data, to delete flies, or to format the hard disk. What Kind of Viruses Are There?There are two main types of viruses：shell,intrusive. Shell viruses wrap themselves around a host program and dont modify the original program. Shell programs are easy to write,which is why about half of viruses are of this type. 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. | 
