A computer virus is a program that can copy itself and infect a computer without the permission or knowledge of the owner.
A virus can pread from one computer to another when its host is taken to the target computer.

A user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive. Viruses can increase their chances of spreading to other computers by infecting files on a network.

The term "computer virus" is sometimes include all types of malware of worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware and other malicious and unwanted software. Viruses are sometimes confused with computer worms and Trojan horses, which are technically different.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses take advantage of network services such as World Wide Web, e-mail, Instant Messaging and file sharing systems to spread it self.

Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS, modem use, and software sharing.

viruses were written to infect popularly traded software. Shareware and bootleg software.

Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel and spread throughout Microsoft Office by infecting documents and spreadsheets.

A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.

Cross-site scripting viruses emerged recently, exploiting websites such as MySpace and Yahoo.

In order to replicate itself, a virus must be permitted to execute code and write to memory.
For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. If a user attempts to launch an infected program, the virus' code may be executed simultaneously.