Worms are very similar to viruses as they are computer programs that replicate themselves often, but do not always contain some functionality that will interfere with the normal use of a computer or a program.

Worms use facilities of an operating system that are meant to be automatic and invisible to the user. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks. While the virus needs an attachment and execution by a user, the worm is a self-sufficient program. It can move through the network unnoticed.