#4. Conficker ($9.1 Billion)
Conficker is, to date, the most sophisticated computer worm ever created. It is also among the most recent in viral threats, causing an estimated 9.2 Billion dollars in damage. Discovered in 2008, conficker used a combination of advanced malware techniques to infect and spread itself. Unlike other malware and viruses, however, the Conficker worm was designed to defend itself from being either detected or removed. Using a fault in networks, it spread rapidly over the internet, using Networks to gain access to large LANs, removable devices and network shares. Infecting an estimated 9-15 million computers worldwide, it used the infected network to push and pull executable payloads and update itself. It is difficult to detect and remove, as it resets system restore points in windows and disables a number of administrative services in windows. Using a predefined list of antivirus services, it matched processes in the computer memory to that list, and stopped them from running. It was responsible for installing malware like spyprotect2009 and Waledec, as spambot.