The Sobig worm is a large emailing worm that sent itself to any email addresses with file extensions like .dbx, .eml, .hlp, .htm, .html, .mht, .wab, and .txt. The worm often used an email that includes "Re:" to seem as if people were being replied to from someone. The user would then open the email and click on the attachment, running the worm and downloading it on to their computer.