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Developments in mail systems, such as Microsoft Outlook, without sufficient thought for security implications, made viruses that indeed propagate themselves via email possible.

Notable examples include the Melissa worm, the ILOVEYOU virus, and the Anna Kournikova virus.

In some cases, a user must open a document or program contained in an email message in order to spread the virus ; in others, notably the Kak worm, merely opening or previewing an email message itself will trigger the virus.