Plagued Residents are just what the name suggests- Civilians across Azeroth having succumbed to the plague and lost their sanity. During the Zombie Infestation World Event, these infected people began wandering around low level areas, attacking any living player or NPC nearby. The infection apparently can make residents insane even if they haven't become Scourge...yet. They come from all walks of life, just like every other walking dead.