I really enjoyed every book of this series! Some of it was really sad and brought tears to my eyes, but mostly it kept me mesmerized with all of the action. There is a very plausible virus that can cause different types of zombies due to the different strains of the virus that cause more or less damage to peoples brains or kills them. Less damage means that the zombies are smart, more damage makes them insane or docile. I highly recommend this series!