This definition explains the meaning of virus. Computer viruses are the oldest forms of malware, dating back to 1971. It works by inserting copies of itself to a program, and spreads when an unwitting user launches it.
A computer virus works in a similar way to a biological virus in that it can attach itself to a program or file and once activated, will spread from one computer to another, infecting them. The code a virus carries could be something relatively benign, such as a virus called Melissa that inserted quotes from the Simpsons into word documents. Or, it is designed to cause serious damage to systems including hardware and software. Activating a virus in your computer system can be as simple as opening a malicious program attached to an executable or host file.