A computer worm is normally designed to infect as many computers as possible using a number of different attack vectors, with each attack vector exploiting a certain weakness which might exist on computers -- for example, Window file shares, emails or buffer overflows in Web servers.
In carrying out the attack, it is common for the worm to replace or modify webpages with its own content so it can spread itself to more computers. The problem with all of this attacking and modifying is that websites and computers will break due to the almost infinite variation of services and webpages. The worm, when patching and modifying a website with its code, can stop the page from being displayed properly or not at all. And when attacking different vectors, it can deny service by using up resources like bandwidth -- or denying access to the service by causing it to fail.
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