A computer virus is a computer program which can copy itself and infect a computer without the owner's permission or knowledge. The term "virus" is also common, but there used to refer to a type of malware, adware, spyware and programs that do not have the ability to reproduce. A true virus can spread only from one computer to another (in some form of executable code) if the host is taken to the target computer, for example, because the user that sent through the network or the Internet, or taken as a removable media such as floppy disk, CD, DVD, or USB drive. The virus can be spread to improve the opportunity by infecting other computers on the network file system file or a file system that is accessed by another computer.

The term "computer virus" is sometimes used as a place for all phrase to include all types of malware. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware and other malicious software and is not desired), including true viruses. Sometimes with computer viruses and worms Trojan horses, which is technically different. A worm can exploit security vulnerabilities to spread itself to other computers without the need to transfer as part of a host, and Trojan horses are programs that appear harmless but have a hidden agenda. Trojans and worms, such as viruses, can cause damage to both the host computer system from the data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have any symptoms to a user's computer, but many are reticent.

Most personal computers are now connected to the Internet and to local area network, facilitating the spread of malicious code. Currently, the virus can also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging, and file sharing systems to spread.