generally replicates itself through an email program by searching for email addresses that it can mail itself to. Once it has located a list of email addresses, it will mail itself in the form of an attachment to each recipient.
The process a worm uses to replicate itself causes the Internet to slow to a crawl due to the massive amount of traffic it creates. It can also gain unauthorized access to a website to launch an attack by sending thousands of information requests to the site in an attempt to crash the site.
Although email is the most common way a worm will replicate itself, it will also attack other applications such as Microsoft Word and Excel by inserting malcode into those applications and then using them as an attachment.