SUCH a great book. I really love Westerfeld's writing, and this is such an interesting take on the vampire myth. Also, every other chapter is devoted to telling the reader about different parasites, not only some that affect humans, but also cows, ants, and a bunch of other unsuspecting creatures.
The vampire myth in this book is totally different from anything I've encountered before (at least for vampires--I've seen my fair share of zombies that are actually infected with parasites or viruses or what-have-you). How it gets transferred from host to host is also interesting, because it stretches beyond the human spectrum.
Cal's mission is to go around collecting Peeps--capturing them and bringing them back to his company. Since he's a carrier, he can't be infected. At some point, I actually felt bad for Lace, because really, she had no choice.
The story eventually moves on to a "bigger than you and me" topic, where the Peeps are actually soldiers meant to SAVE the human race from something bigger. We never do actually get to the final battle part (I'm hoping it shows up in the sequel, which I haven't read yet), but the build-up to it is really good and kept me hanging on the whole way.