What is Virus?

Definition of Virus:-

A Virus is a small program that embeds itself into other programs. When those other programs are executed, the virus is also executed, and attempts to copy itself into more programs. In this way, it spreads in a manner similar to a biological virus.
viruses, by definition, can "infect" any executable code. Accordingly, they are found on floppy and hard disk boot sectors, executable programs, macro languages and executable electronic mail attachments.
viruses can be found using a Virus Scanner or a Virus Wall. Some software products are also available to remove them with a minimum of harm to the "infected" files.
Some viruses are self-modifying, in order to make detection more difficult. Such viruses are called polymorphic.

A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves. All computer viruses are manmade. A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems.

Dr. Cohen's definition of a computer virus as "A program that can 'infect' other programs by modifying them to include a ... version of itself" is generally accepted as a standard. On occasion it presents problems with the acceptance of, say, boot sector viral programs and entities such as the Internet/UNIX/Morris worm. However, his work did experimentally demonstrate and theoretically prove many vital issues.