Worm
Self-copying hostile software that uses the network as a tram. Often no click required once it is inside.
A worm copies itself to other devices across a network, often without a user doing anything. It does not need to glue itself to an existing program.
This is why unpatched internet-facing services and flat networks still end careers. Segmentation, rapid patching, and turning off unused services shrink the tram lines.
Fact source: ASD's ACSC glossary.