Computer viruses date back to the 1960's when Xerox researchers noticed that when programs were run simultaneously, instructions from one program ended up in another program, creating holes in the programming code in the same way a worm eats through an apple. There are currently over 45,000 known viruses and almost a dozen new ones created every day. About 500 of these viruses are outside the hands of research laboratories, meaning they are in the wild. Introduction to Viruses. 13 Jan 2000.
Computer viruses spread in three general ways.
The first is a boot sector virus, a set of instructions the computer reads when it starts up. A boot sector virus first infects a floppy disk and then replicates on the boot sector of the hard disk. Boot sector viruses lost popularity in the 1990s, when boot disks became mostly obsolete. The second is a program virus infecting executable files like .EXE and .COM files. Once an infected executable file is started, the virus usually replicates into the computer's memory.