It's only a little package of data. Maybe 2000 lines of computer code, a barely noticeable drop of water in the ocean of your computer. But the Conficker worm, once it's wormed it's way in, can hijack your system and use your processing power for criminal activities. Conficker is already inside millions of computers around the world, possibly yours even though you don't know it, and it's all being controlled by an anonymous bad guy who hasn't been found.
This sounds like science fiction but it's actually happening. We talk to Mark Bowden of The Atlantic about Conficker. He explains how the worm, despite it's broad reach, has never really been activated but if that secret bad guy chooses to flip the switch and wake Conficker up, it could form a sort of computer zombie army that could take down the online systems of companies, banks, even governments.