similar thing happens in the Expedition to Castle Ravenloft. Where the PCs come to the village of Barovia to find out that everything is boarded up and most of the town's inhabitants have became zombies. It is the goal of the PCs to find the cure to the problem. This is how it works there.
Desease (Su) Necromantic infection --slam, Fort DC 13negates, incubation period instant, damage sickened. An infected creature that drops to -1 hit points or fewer, or that dies, rises as a zombie in 1d4+1 rounds unless properly treated.
There is a way to treat it, but I doubt most NPCs will have the means. This would be very taxing on the MLs as those who become infected would not know where to turn. Can turn the whole village or even Vallaki into a place filled with walking corpses that others keep joining their ranks. Very deadly to low level PCs.
Could use this scene as a DM event and grab a party to solve the case. I suggest going away from Vallaki or Barovia and make it in a village which is difficult to get to.