A program that replicates itself by being copied or initiating its copying to another program, operating system, or document. Viruses are transmitted in many ways, such as in attachments to e-mails, as part of downloadable files, or be present on diskettes or CDs.

Some viruses wreak their effect as soon as their code is executed; other viruses lie dormant until circumstances or events cause their code to be executed by the unsuspecting host.