A、They invade the computer and make it a place for producing viruses. B、They spread throughout the whole system by copying thems

The computer virus is an outcome of the computer fast growth in the 1980s.(22)The cause of the term "computer virus" is the likeness between the biological virus and the evil program infected with computer.
    The origin of this term came from an American science fiction written by Thomas Ryan, published in 1977. Human viruses invade a living cell and turn it into a factory for producing viruses. However, computer virus are small programs.(23)They expand 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. 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 level of the user, and the number of times a file is used. Likewise, the mode of attack varies.(24)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.(25)Bad viruses are designed to damage the system. The attack is to wipe out data, to delete flies, or to format the hard disk.