The novel is labelled as "Young Adult", but I'd consider it simply "easy reading" (or does YA just mean "no sex" these days?)
The ending was a little too much like the end of the Will Smith I Am Legend
movie for my taste. Otherwise I found it quite enjoyable, and read it in one sitting.
It's a fairly typical "[b:Lord of the Flies|7624|Lord of the Flies|William Golding|http://ecx.images-amazon.com/images/I/51VaoqPM%2BUL._SL75_.jpg|2766512]" scenario, where we can expect civilization to crumble particularly quickly when only the very young survive the apocalypse, but Aguirre rather ruins this with her afterword in which she quotes a number of highly speculative websites - and one scientific paper which used popular media as a source for data on modelling a zombie outbreak! My advice, just don't read the afterword - there's nothing wrong with most of the story without the pseudo-scientific background.