A worm can replicate between machines through computer networks. Usually, worms take advantage of a security hole in software or an operating system. They can also be installed by downloading files to a computer.
One example of a worm is CODE RED. It only replicated during the first 20 days of each month--a feature to make it more difficult to discover the worm. It also replaced web pages on servers which had been infected with a page that said "Hacked by Chinese." Along with this, it attempted to overwhelm the White House Web Server, but the White House changed its IP address--a specific number assigned to the its server--successfully avoiding the problem.
