But first, let's start with a motivating example, a serious problem: how do we stop computer worms?

Worms have tremendous economic impact, even when all they do is cause downtime and don't actually damage any data. You've probably heard of worms like Code Red (IIS), Sasser (lsass), or the Witty worm (ISS). Worms are getting faster and scarier as we rely more and more on computers -- the time between the discovery of a remotely exploitable vulnerability and an actual attack is shrinking.