A computer virus is a self-replicating computer program or a segment of code that inserts copies of itself into other programs, thus infecting them. As a result, the original programs cannot run smoothly, thereby totally disrupting the functioning of the computer.
When an infected program is run on a computer, the hidden virus is activated and attempts to inject itself into additional programs. This process is analogous to the biological process of virus spreading from cell-to-cell within an organism. Very much like the biological virus, a single computer virus can spread to any number of compatible computer systems if provided with a pathway for infection.
The virus infects a system through sharing of infected diskette or through communication links, like local area networks or programs transmitted from distant systems.
Some of the commonly destructive manifestations of a computer virus include erasure of recorded data, rendering of entire device drives unreadable, interfering with communication and breaking the security cordon of the host computer. The virus may also create other problems such as display of unusual messages or plotting the text backward. Some of the well-known viruses are Pakistani Brain, Lehigh, Friday the 13th, Christmas and Bloody.