Conficker

In November 2008, a worm quietly sounded the geek alarms as it set about to infect millions of personal computers around the internet -- to date, estimated at 6-7 million. It was christened Conficker by MS security staff. It exploited a flaw in MS Windows operating systems -- got into the computer, closed the flaw, and went about replicating itself. It communicates to its home base with an encryption level that is so powerful, it's still in the lab -- and is one of the contenders for a new security standard to be published in 2013. It is part of a powerful botnet that was used only once, in what looks to have been designed to test its viability. Ever since then, it has sat dormant. Waiting. Not much is known about it's purpose or who controls it. What is known however is scary, because there is no way to stop it, and it can be used to cause severe disruption if needed.

The good guys put up a good fight, but may have lost to the bad guys on this one -- but no one knows what the cost of losing is.

in reference to:

"The key word there is could, because so far Conficker has done none of those things. It has been activated only once, to perform a relatively mundane spamming operation—enough to demonstrate that it is not benign. No one knows who created it. No one yet fully understands how it works. No one knows how to stop it or kill it. And no one even knows for sure why it exists."
- The Enemy Within - Magazine - The Atlantic (view on Google Sidewiki)