Today we went to Kill some Alliance bosses, and all 4 of them are immune to Diseases, tested on DKs with 3 diseases and on a shadow priest with dev.pleague..everything else works normaly on them.
Its just that they are immune.
Edit: Screenshot 1 : http://i58.tinypic.com/znoj09.jpg
You can clearly see that I use Icy Touch and Pleague Strike and The king doesn`t recive diseases
Screenshot 2: http://i62.tinypic.com/33lyref.jpg
You can see that he;s immune to pestilence, if a mob/npc doesn`t have diseases on him pestilence just uses 1 blood rune and does nothing.
PS: don`t lock this thread i`ll update it with other elite mobs that immune.
-Vshu