A computer virus is a essentially a computer program also known as computer software that can copy itself and infect computers without permission or knowledge of the user.

The virus may modify itself while copying itself (infecting) or the copies may modify themselves, while infecting others, in a manner quite similar to a metamorphic virus (a biological virus that can change on its own).
A virus can only spread from one computer to another when data is shared between the two. In earlier times, the most common method of spreading was via floppy disks. These days, it may also spread via other removable medium such as CDs, DVDs, Thumb / Pen drives, removable hard disks, tapes etc. A virus can also spread via file sharing on computer networks, peer-to-peer networks, emails, and even web sites.