Definition: A Virus is a threat. It's a self-replicating malicious computer code that is designed to intrude upon and damage a computer, network or other components of an information infrastructure.
A virus may arrive from a computer or computer implement that is already infected. This class of threat can also be on media such as a USB drive or the Internet. When an infected secondary storage device is inserted into a "clean" computer and the program is executed the malicious software is copied into the target and it becomes infected, too.
The characteristics of software that infect computers or computer networks have certain 'signatures' and publishers of anti-virus software eventually adapt so that can counter the threat.
Its Relevance: Information systems must be protected from all malicious software threats. The organization is, otherwise, being negligent. The manner in which malware is handled should be specified in the organization's information security policies and procedures.