Definition:

A virus is a software program, usually designed to cause harm, that is installed without your knowledge. A virus cannot spread on its own. It requires an individual to initiate an action, such as opening an infected email attachment, before it can run and propagate. Once activated, a virus can make copies of itself and spread to other systems, usually by attaching itself to existing files.

Examples: One of the most common methods of spreading a virus is through an email attachment.