When Cal Thompson loses his virginity in a one-night-stand, he's left infected with a parasite that makes him ravenous for red meat and has souped up his senses. As a carrier, he can control his urges, but others aren't so lucky and are driven to kill and maim people as they seek to spread the parasite to others, while things that they previously loved have now become anathema to them.
Cal joins the Night Watch, a secret organisation that seeks to control the spread of the parasite (which is transmitted through kissing and sex). Now, Cal has to track down his subsequent girlfriends, all of whom have been infected, including his one-night-stand, whose name he can't remember. When Lacey, a journalism student, tells him about weird things happening in her apartment block, Cal discovers that the parasites are only part of the problem - there's something big and evil lurking beneath New York City, something that only Cal and the Night Watch can stop.
Westerfeld's use of parasites is an interesting take on vampirism and by interspersing the action with scientific interludes explaining how parasites work in nature, he gives his worldbuilding added credibility. However while we start with the idea of the infected all being homicidal maniacs, the only infected characters we meet are those who retain some element of control. The distinction between `zombies' (infected and unable to control it) and `vampires' (infected and able to control it), doesn't come until the end of the book which was confusing, not least because the `zombies' are never shown.
Cal is an engaging (if at times too naive) hero although he takes longer than the reader to work out what's really going on. The problems of being a carrier are nicely depicted - notably his sexual frustration. Lacey is a good counterfoil, curious to know what's really going on and smart enough to work out when she's being lied to. However other characters are little more than cyphers used to keep the plot moving - notably Dr Rat, the Professor and the mysterious Morgan.
There are some very tense scenes - the opening, with Cal tracking down a former girlfriend is gripping but, the final revelations are anti-climactic exposition dumps. There's a set-up for a sequel, which I would be interested in reading, but I hope that there's more clarity as to the threat that Cal is facing.