Six years ago, Sally was in an accident that should have killed her. But she woke up, and SymboGen is attributing her miraculous recovery to the "implant"--a genetically modified tapeworm--in her system. But the implants are starting to have unforeseen consequences, and Sally begins to learn horrible things about the history of SymboGen and their miracle cures. 
 Grant has an interesting way of writing her zombie apocalypses (apocalypi?) in such a way that the characters rather than the crisis are the center of the tale, and yet the crisis turns out to be the impetus for everything the characters do. I'm not sure I'm explaining it well; perhaps it would make more sense to say these stories are character driven rather than plot driven, yet the plot doesn't take as much of a backseat as it tends to in character driven stories. 
 Sally is an interesting character who serves as reader analogue--waking up as a blank slate means that she needs to learn things about the world the same way the reader does--and yet is a major, central part of the "sleepwalking" crisis and the biological horrors revealed toward the end of the book. The twist at the end is something that simultaneously teases at the edges of your consciousness as you're learning more about the mad science and is as much a surprise to the reader as it is to Sally.