Every once in a while, an area on the map that is not highly trafficed (basically, any place that\'s not a road), could be designated as an infestation area. Basically the spawn rate for a specific type of monster slowly increase over time (say... over a week). Once it reaches a certain points, it could become a city-sponsored quests to clear out said area (bounty hunting, or for guilds, kill a bulk number of monsters for a slightly higher rate). Said infestation could reduce over time.
Here are basically the stages of an infestation.
1. Incubation (1 ~ 2 weeks) - Increases in monster spawn rate.
2. Outbreak (1 ~ 2 weeks) - Monster spawn rate maintained, quests given at this point.
3. Cooling (variable, depending on how many monster killed during the outbreak period) - Monster spawn rate reduce of normal.