A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The original may modify the copies or the copies may modify themselves. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable medium such as a floppy disk, CD, or USB drive. Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer.

       The term comes from the term virus in biology. A computer virus reproduces by making, possibly modified, copies of itself in the computer's memory, storage, or over a network. This is similar to the way a biological virus works

        Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behaviour and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.

          A program called "Elk Cloner" is credited with being the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created. The first PC virus was a boot sector virus called (c)Brain, created in 1986 by two brothers, Basit and Amjad Farooq Alvi, operating out of Lahore, Pakistan.