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－1“　written  by Thomas J. Ryan，published in 1977．Human viruses invade a living cell and turn it into a factory for manufacturing viruses. However, computer virusesare small programs. They replicate by attaching a copy of themselves to another program.    Once attached to me host Program，the viruses then look for other programs to "infect"．In this way， he 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 IBM’s main computer system last Christmas with a season's 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 four main types of viruses: shell, intrusive, operating  system  and  source  ode．     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．     Intrusive viruses Invade an existing program and actually insert a portion of themselves into the host program．Intrusive viruses are hard to write ad very difficult to remove without damaging the host file.     Shell and intrusive viruses most commonly attack executable program flies－those with a．corn or．exe extension， although data flies 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．FOF example，some operating system viruses have hidden 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 program such as those written in Pascal prior to the program being complied．There are the least－common viruses because they are not only hard to write，but also have a limited number of hosts compared to the other types． Be Wary of the Second Network Viruses     Do you believe it？ Network viruses can steal money！So far Internet has become the main channel through which the computer viruses spread．Look，here come the second network computer viruses．Even without "snatching" information from  the network your computer can be infected by the second network computer viruses， which are hidden in some machines on the network． Your computer is， so to speak， in danger once being connected to the network to browse．     The virus that can steal your money belongs to a kind of the second network viruses.  It was designed and put in some machines on the network． When your computer is linked to one of these machines，the virus will invade your hard disk and search whether Intuit Quicken， an accounting software， is installed． One of this accounting software's functions is to transfer accounts automatically. Once infected， your money will be transferred to an additional account opened by the virus program without anybody knowing 
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