Nimda Virus (Concept Virus)
Otherwise known as the Concept virus, the Nimda virus infected hundreds of thousands of machines in 2001. Unlike the typical email attachment virus that required manual launching, Nimda spread as an embedded attachment in an HTML email message that would execute as soon as the recipient opened the message. The worm modified Web documents and executable files, and like most worms, replicated itself on infected machines. Through direct server-to-server transfer, it infected shared hard drives on networks. The virus was also spread to personal computers when users' browsed Web pages hosted on infected servers.
Nimda soon inspired authors to create a crowd of imitators that followed the same pattern.