Definition of 'Computer Virus ?

Definition: A computer virus is a malicious software program loaded onto a user’s computer without the user’s knowledge and performs malicious actions.

Description: The term 'computer virus' was first formally defined by Fred Cohen in 1983. Computer viruses never occur naturally. They are always induced by people. Once created and released, however, their diffusion is not directly under human control. After entering a computer, a virus attaches itself to another program in such a way that execution of the host program triggers the action of the virus simultaneously. It can self-replicate, inserting itself onto other programs or files, infecting them in the process. Not all computer viruses are destructive though. However, most of them perform actions that are malicious in nature, such as destroying data. Some viruses wreak havoc as soon as their code is executed, while others lie dormant until a particular event (as programmed) gets initiated, that causes their code to run in the computer. Viruses spread when the software or documents they get attached to are transferred from one computer to another using a network, a disk, file sharing methods, or through infected e-mail attachments. Some viruses use different stealth strategies to avoid their detection from anti-virus software. For example, some can infect files without increasing their sizes, while others try to evade detection by killing the tasks associated with the antivirus software before they can be detected. Some old viruses make sure that the "last modified" date of a host file stays the same when they infect the file.