David L Smith, 33, pleaded guilty to charges of computer theft and sending a damaged computer program.
The Melissa virus flooded corporate networks with e-mail messages. It affected a third of all company e-mails in the UK and caused the closure of e-mail systems of government agencies in both the US and UK. In the US it forced some companies, including Intel and Microsoft, to shut down their e-mail servers.
The business cost of Melissa is hard to quantify but in court, Smith and prosecutors agreed that the damages in the case were greater than £55m. Some estimates put the global cost at more than £800m.
The macro virus launched when a user opened an infected Microsoft Word document sent as an e-mail attachment.
The e-mail, usually bearing the name of someone the recipient knew, had a subject line that said: "Here is the document you asked for... don't show anyone else ;-)." When a user opened the attachment the virus was sent to the first 50 names in their address book.