This is a little explanation of the difference between a worm and a virus. There is a difference between the two kinds of "malware", although some virus/worms blur the line somewhat. It is also true that the media is often confused about the difference.
A virus is a piece of software that lurks in another piece of software until it is activated (by a user running it) whereupon it can do anything that any other piece of software can do. Older and more destructive viruses could wipe your hard disk and make it impossible to re-build the machine!
Worms on the other hand do not need to be activated by a user. They make use of vulnerabilities in software like Outlook (or to be fair plenty of other software) so that they can automatically activate --- their code becomes part of the running program. Once running, they can do similar things to viruses, although they are quite often a bit more limited.
In the past, worms were rare and slow spreading problems, but Microsloth's totally inept programming has meant that they are now very widespread, and can spread across the Internet very rapidly indeed.
It is quite common for mail worms that are self-activating in packages such as Outlook to work as conventional viruses if the user who receives them activates the attachment.