I'll talk about :
-Necrotic Plague - Spell - World of Warcraft : Disease put by Arthas during his fight (phase 1 : 100% to 70%)
and
-Pestilence - Spell - World of Warcraft : DK's skill + Glyph of Pestilence - Item - World of Warcraft (glyph)
So you when you fight against the Lich king you will see some Necrotic Plague in the raid, if the player is dispelled when his closest "unit" is a mob, the plague will jump on this mob.
Now the bug :
If the plague is on a DK, and the DK got dispelled when his closest unit is a mob, the desease is consired as his own.
He can refresh her using Pestilence, the mob will keep the desease all the time! So you don't need to put all dps on him, you can focus Arthas and you don't have any "dps" issue during this phase.
You can see a DK using it on this video :
[ame=http://www.youtube.com/watch?v=rbN5ZklPFsI]YouTube - Adept vs The Lich King (1 of 2)[/ame]
(the best example is a 2:20 : the desease is going to fade, and the DK refresh her using pestilence)
PS : Sorry for my poor english, anyway i hope it can help you