2.2.1 What is the difference between a worm and a virus?
Unlike a virus, a worm does not infect files on a host computer. Instead it adds a file to the computer that is malicious code, and runs it 'in the background'. A computer has many programs running in this way in order for its system to operate. For instance, when you create a document you can see the text editor, such as Microsoft Word, Notepad or Star Office, but in the background the spell checker or the printer program are working even though you do not see them on the screen.
Worms can spread through any medium that is being used to connect to the Internet, whether it be a modem, broadband, wireless connection, or a local area network at work.
The website of the antivirus software vendor Sophos describes W32/Netsky-R, a worm that was first seen in March 2003. This worm is part of a 'turf war' that was being waged at the time of writing between the writers of Netsky and another worm called Bagle. You don't need to worry about the technical details included in the description.