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 humans have become. Microsoft and other large companies had to shut down all their email systems when the “Melissa” virus became a worldwide event. A computer virus is transmitted from one computer to another. A virus must overlap with some other program to be documented in order to execute an instruction. After being executed, it can infect other programs. Viruses were first spotted in the late 1980s; the first factor was the spread of personal computers. Before the 1980s home computers were either non-existent or used as toys, and real computers were very rare and were locked away to be used only by "experts". The second factor was the use of "bulletin boards"; anyone could access a bulletin board if they had a modem and download programs. Bulletin boards led to the precursor to the virus known as the Trojan horse. It's a program that sounds really cool when you read it, then people download it, and when people run the program, however, it does something uninteresting like erasing your disk, so people think they're getting something clean, but it erases their system. The third factor that causes viruses is the floppy disk. The programs were small and could put the operating system, or a word processor on 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 such as games or word processors. People might download an infected game from a message board and run it, and a virus like this is a small piece of code embedded in a larger legitimate program. The virus loads itself into memory and looks around to see if it can find any programs on the hard drive. When it finds one, it changes it to the virus code in the program. The virus then launches the “real program” and the user has no way of knowing if they are infected. The next time this program runs, they infect other programs and the cycle continues.