https://www.quora.com/What-is-the-most-sophisticated-piece-of-software-code-ever-written/answer/John-Byrd-2

This worm exists first on a USB drive. Someone could just find that USB drive laying around, or get it in the mail, and wonder what was on it. When that USB drive is inserted into a Windows PC, without the user knowing it, that worm will quietly run itself, and copy itself to that PC. It has at least three ways of trying to get itself to run. If one way doesn’t work, it tries another. At least two of these methods to launch itself were completely new then, and both of them used two independent, secret bugs in Windows that no one else knew about, until this worm came along.

Once the worm runs itself on a PC, it tries to get administrator access on that PC. It doesn’t mind if there’s antivirus software installed — the worm can sneak around most antivirus software. Then, based on the version of Windows it’s running on, the worm will try one of two previously unknown methods of getting that administrator access on that PC. Until this worm was released, no one knew about these secret bugs in Windows either.