Worm Virus's

[Potatoman]

So lets assume i have a worm on my PC that sends itself to everyone in my address book. Apart from infecting the recipients, whats the major worry of worms if all they do is send themsleves and dont damage files?

[Whickerman]

Firstly, they can indeed contain harmful viruses, which can sit in your pc and ruin it later, or leave spyware etc.

Secondly, imagine what happens when one gets into, say, a business network, with hundreds or even thousands of addresses. The entire network gets loaded with mails to everyone, which then reply to everyone at once, and then the replies reply... see where this is going? Entire businesses can stop functioning in a very small space of time.

[stevie21]

Yep - totally clogs up t'internet.

At worst, if it wasn't for all the traffic created by spam and worms then the web would be quite a quiet place and ISPs may not even make too much fuss about metered and unmetered connections.
A more accurate and realistic rendition is that the biggest impact is as Whickerman says and it cripples internal networks.