In all it is a good book, but there are some major downsides to it, in my opinion.
The book starts very good. You are immediately taken into the situation without a very long introduction. Also the rest of the action story around Mark Bryan is very good. It's not just the "bad guys" that get killed.
Downside however is that the ending is VERY rushed! It feels to me like important parts of the story are just missing. I would have prefered the book to be 50 pages longer and to have some better plot twists than the way it is now. Also, Dr. Turner and the Walkers (the sarge and his wife Emma) are very 2-dimentional. You don't get any insight in their caracters or why they are doing what they are doing.
So for me, it brings the book down from a certain 5 stars to just 3 stars.