Deathknights have an ability called Pestilence that spreads diseases between targets.
Shadow Priests have a disease called Devouring Plague that can be spread around by a Deathknight.
I had a brief discussion with one of our death nights and the trick though is to put your disease on the target of the deathknights before he casts pestilence, Its not often in a trash pull that they need to recast it, so you HAVE to have it up before. From my understanding it is possibly the 3rd spell cast in their rotation, depending if they are tanking or dpsing.
When the DK spreads the love your disease will act as another AOE spell. So it will be well worth targeting that deathknights target and sticking on Devouring Plague – and then mind sear agro dump mind sear.. blahhh..
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Pugnacious Priest's Warcraft Blog 