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.
Some viruses are self-modifying, in order to make detection more difficult. Such viruses are called polymorphic (many shapes).