Virus
Malware that attaches to a host file and spreads when the host is executed.
A virus attaches itself to a host file or program and spreads only when that host is executed — so unlike a worm, it requires user action to propagate. Common variants include boot-sector (infects startup code), macro (rides in Office documents), and polymorphic/metamorphic viruses that mutate their code to evade signature detection. Behavior-based EDR and disabling document macros are key modern defenses.