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The computer virus is an outcome of the computer overgrowth in the 1980s.The cause ofthe term "computer virus" is the likeness between the biological virus and the evil program infected with computers.The origin of this term came from n American science fiction "The Adolescence of P- 1”written by Thomas J.Ryan, published in 1977.Human viruses invade a living cell and tun it into a factory for manufacturing viruses.However, computer viruses are small programs.They replicate(复制) by attaching a copy of themselves t 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 theuser, 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 IBM's main computer system last Christmas with a season's getting.Malignant viruses are designed to damage the system.The attack is to wipe out data, to delete flies, or to format the had disk.There are four main types of viruses: shell, intrusive, operating system and source code.Shell viruses wrap(包裹) 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.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.Shell and intrusive viruses most commonly attack executable program files- those with a com or exe extension, although data files are also at some risk.Operating system viruses work by replacing parts of the operating system with their own logic.It is very difficult to write operating system viruses and these viruses have the ability once booted up, to take total control of your System.For example, some operating system viruses havehidden large amounts of attack logic in falsely marked bad disk sectors.Source code viruses are intrusive programs and they are also inserted into a source programsuch as those written in Pascal prior to the program being compiled.There are the least commonviruses because they are not only hard to write, but also have a limited number of hosts compared to the other types.