This keeps getting better!
This is the 3rd book in this series. These books, to be thoroughly enjoyed, must be read in order as they continue a story.
The premise without spoilers: In Colorado Springs, for some mysterious reason, certain people begin to laugh for no reason. By itself, it's not really a big deal. However, accompanying this behavior is that they enjoy killing while they laugh. Meanwhile, regular folk need to get away from these people or else they become their victims.
The real scary part? It seems to be viral, so one could start out normal and then end up laughing.
Great Series as well as different as far as apocalypse plots go.
Highly recommend!