From the introduction:
The undead have been drifting into town for weeks. At first they shuffled towards the central square in groups of two or three. But as time passed they began to gather and move in larger groups of a dozen or more. The town watch has had only minor problems with the undead, and only a handful of townsfolk have been killed or bitten and become undead themselves.
Unlike most undead, these walking corpses appear to chuckle or laugh at those they fight or scare away. The townspeople have become scared at the thought of what force is sending them this seemingly endless tide of laughing zombies, and as the party enters the town they are hired to investigate and rid them of this menace.