A standalone program that exploits an unpatched network service to copy itself from machine to machine with no user action at all.

A program advertised as a free video converter that installs normally but quietly opens a remote backdoor; it never replicates itself.

Code that embeds itself at high privilege and conceals its own files and processes from the operating system and from the user.

Code embedded inside an existing file that runs only when the user opens that file, and then copies itself into other files.