Virus
Hostile software that hitchhikes: it attaches to other programs or copies itself across systems. Needs a host. Still not a metaphor for every pop-up.
A virus is malware that spreads by attaching itself to other programs or by copying itself between systems and networks. It needs something to ride.
People say 'virus' for every nasty file. Be precise when it matters: worms self-travel, Trojans impersonate, ransomware bills. The response still starts with isolate, preserve if you must, and rebuild from known-good.
Fact source: ASD's ACSC glossary.