VIRUS HISTORY

Traditional computer viruses were first widely seen in the late 1980s, and they came about because of several factors. The first factor was the spread of personal computers (PCs). Prior to the 1980s, home computers were nearly non-existent or they were toys. Real computers were rare, and they were locked away for use by "experts." During the 1980s, real computers started to spread to businesses and homes because of the popularity of the IBM PC (released in 1982) and the Apple Macintosh (released in 1984). By the late 1980s, PCs were widespread in businesses, homes and college campuses.
The second factor was the use of computer bulletin boards. People could dial up a bulletin board with a modem and download programs of all types. Games were extremely popular, and so were simple word processors, spreadsheets and other productivity software. Bulletin boards led to the precursor of the virus known as the Trojan horse. A Trojan horse is a program with a cool-sounding name and description. So you download it. When you run the program, however, it does something uncool like erasing your disk. You think you are getting a neat game, but it wipes out your system. Trojan horses only hit a small number of people because they are quickly discovered, the infected programs are removed and word of the danger spreads among users.
Floppy disks were factors in the spread of computer viruses.
The third factor that led to the creation of viruses was the floppy disk. In the 1980s, programs were small, and you could fit the entire operating system, a few programs and some documents onto a floppy disk or two. Many computers did not have hard disks, so when you turned on your machine it would load the operating system and everything else from the floppy disk. Virus authors took advantage of this to create the first self-replicating programs.Early viruses were pieces of code attached to a common program like a popular game or a popular word processor. A person might download an infected game from a bulletin board and run it. A virus like this is a small piece of code embedded in a larger, legitimate program. When the user runs the legitimate program, the virus loads itself into memory­ and looks around to see if it can find any other programs on the disk. If it can find one, it modifies the program to add the virus's code into the program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time the user launches either of those programs, they infect other programs, and the cycle continues.
If one of the infected programs is given to another person on a floppy disk, or if it is uploaded to a bulletin board, then other programs get infected. This is how the virus spreads.
The spreading part is the infection phase of the virus. Viruses wouldn't be so violently despised if all they did was replicate themselves. Most viruses also have a destructive attack phase where they do damage. Some sort of trigger will activate the attack phase, and the virus will then do something -- anything from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date, the number of times the virus has been replicated or something similar.
In the next section, we will look at how viruses have evolved over the years.