A final reply comes from Jacob Motola of Integralis, a software security company:
The concept behind the first malicious computer programs was described years ago in the Computer Recreations column of Scientific American. The metaphor of the "computer virus" was adopted because of the similarity in form, function and consequence with biological viruses that attack the human system. Computer viruses can insert themselves in another program, taking over control or adversely affecting the function of the program.
Like their biological counterparts, computer viruses can spread rapidly and self-replicate systematically. They also mimic living viruses in the way they must adapt through mutation to the development of resistance within a system: the author of a computer virus must upgrade his creation in order to overcome the resistance (antiviral programs) or to take advantage of new weakness or loophole within the system.
Computer viruses also act like biologics in the way they can be set off: they can be virulent from the outset of the infection, or they can be activated by a specific event (logic bomb). But computer viruses can also be triggered at a specific time (time bomb). Most viruses act innocuous towards a system until their specific condition is met.
The computer industry has expanded the metaphor to now include terms like inoculation, disinfection, quarantine and sanitation. Now if your system gets infected by a computer virus you can quarantine it until you can call the "virus doctor" who can direct you to the appropriate "virus clinic" where your system can be inoculated and disinfected and an anti-virus program can be prescribed.
Answer originally posted September 2, 1997.