#1. MyDoom ($38.7 Billion)
The most expensive virus in the world in terms of monetary damage was MyDoom. MyDoom did an estimated $38.5 billion USD in economic damage. MyDoom came in January of 2006, and as of January 2004, it was the fastest spreading virus ever created. MyDoom is believed to have originated in Russia, but the author is still unknown.

It’s believed that MyDoom is a variant of MiMail. Mainly transmitted by e-mail, appearing as an error similar to what a user would get if their mail failed to send. The user would unwittingly open the attachment in the e-mail and the worm would re-send itself to every address it could find. The original version contained a payload that did two things: it opened a backdoor into your computer, allowing remote control of your computer, the other, was to perpetrate a DDOS(Direct Denial Of Service) against SCO group’s website.
The other version, MyDoom.B along with the above payload, also contained malicious software that blocked access to Microsoft and Antivirus websites to prevent the user from attempting to remove the virus.

To this Day, MDdoom is still in an active state, re-appearing in 2005 and recently in the 2009 DDOS attacks in South Korea and the United States.