(worded carefully to avoid main quest spoilers).
Currently, if you complete the available Curing the Plagued quests before starting the Culling the Plagued quests, you cannot complete the Culling the Plagued quests after (20)A Darker Threat.
This is because after completing (40)Cause and Effect of the Curing the Plagued quest book, you cannot interact with the necessary NPC you need to interact with for (20)A Darker Threat.
Edit: The workaround currently to this is to have someone who has not yet done the quest summon you to the necessary NPC.
Edited by huhbum, 08 January 2014 - 23:18.