What is a computer virus?

A computer virus is a computer program that can copy itself and infect a computer without our permission or knowledge. The term "virus" is commonly but erroneously used to refer to other types of malware, adware & spyware programs that don’t have reproductive abilities.

• A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a CD, DVD, or USB drive.
• Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.

Viruses are very different (technically) from COMPUTER WORMS & TROJAN HORSES:

• A WORM can use security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host
• A TROJAN HORSE is a program that appears harmless but has a hidden agenda. Worms and Trojans, like viruses, may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have symptoms noticeable to the computer user, but most are hidden. This makes it hard for most users to notice, find and disable and is why anti-virus programs are now commonplace