Lunacy was believed to be an affliction where the moon's face drove the weak-minded into fits of madness. Behavior associated with menstrual cycles also led to the idea of 'hysteria' or madness caused by internal organs. A village could be normal every time the adventurers pass through, but rumors might persist: a bare dozen travelers per year disappear on the road through this village.
Vibrio Vulnificus (and other Vibrio infections) are spread by the bites of reptiles such as alligators and would be attributed to poison or venom. The wound site turns black, green, or purple and swells. Skin near the wound develops pustules and boils, and the victim runs a high fever. Without treatment the limb might fall off -- but the time frame is long enough to spur a "save the fighter's arm!" side quest. Kobolds, dragonkin, drakes, and other reptiles could spread disease instead of generic 'poison damage' if they live somewhere particularly septic like a swamp. Go to Comment
The local band of 'nasties' (goblins / orcs / whatever) lives in relative peace with the local population.
Along come the PCs and go through their usual heroic monster bashing routine, wiping out the nasties and pinching all their stuff, then continue on their way.
Problem is, they don't kill ALL he nasties. The survivors want revenge and, after spending a few months recovering, start to take it in their own inimitable style (which is not unlike that of the PCs come to think of it).
Next time the PCs are in the area they find themselves VERY unpopular with the townsfolk.