How Computer Viruses Work Computer viruses are not very well understood, but they get your attention. Viruses show us how vulnerable we are, but they also show how open and worldly human beings have become. Microsoft and other major companies had to shut down all of their email systems when the "Melissa" virus became a global event. A computer virus is transmitted from one computer to another. A virus must overlap with another program to be documented in order to execute an instruction. Once executed, it can then infect other programs. The viruses were first observed in the late 1980s; the first factor was the spread of personal computers. Before the 1980s, personal computers were non-existent or used for toys, and real computers were very rare and they were locked down for use only by "experts". The second factor was the nature of the “bulletin boards”. ; anyone could access a bulletin board if they had a modem and downloaded programs. The bulletin boards led to the virus precursor known as the Trojan horse. It's a program that looks really cool when you read it, so people download it, and when people run the program, however, it does something uncool like erase your disk, so people think that 'They get something interesting, but it wipes their system. The third virus driver is the floppy disk. The programs were small and could put the operating system or a word processor onto the floppy disk, then turn on the machine and load the operating system and everything else from the disk. Viruses took advantage of these three facts to create the first self-replicating programs! The first viruses were pieces of code attached to programs like games or word processors. People can download an infected game from a bulletin board and run it, and a virus like this is a small piece of code embedded in a larger legitimate program. The virus loads into memory and looks around to see if it can find any programs on the hard drive. When it finds one, it modifies it with the virus code in the program. The virus then launches the "real program" and the user has no way of knowing that he is infected. The next time this program is run, they infect other programs and the cycle continues.