A computer virus is a computer program that replicates itself and spreads from one computer to another. Viruses increase their chances of spreading to other computers by infecting files on a network file system.

Many people erroneously use the term "virus" to refer to other types of malware, and they include in the same category adware and spyware programs that do not reproduce themselves.

A true virus spreads from one computer to another (in some form of executable code) when a computer user takes its host to the target computer; for instance because a user sends it over a network or the Internet, or carries it on a removable medium such as a floppy disk, CD, DVD, or USB drive.

Malware includes computer worms, Trojan horses, most rootkits, spyware, dishonest adware and other malicious and unwanted software, including true viruses. People sometimes confuse viruses with worms and Trojan horses, which differ technically. A worm exploits security vulnerabilities and it spreads itself automatically to other computers through networks, while a Trojan horse is a program that appears harmless but hides malicious functions.

Worms and Trojan horses, like viruses, can harm a computer system's data or performance. Some viruses and other malware have symptoms noticeable to the computer user, but many more do not do anything to call attention to themselves. Some viruses do nothing more than reproduce themselves.