Aside from Undead Priests and their Devouring Plague, there are no player abilities that inflict the disease Dispel Type. So far, two disease-based abilities have been revealed: Blade Strike and Blood Strike. Designed to work off each other, Blade Strike is an attack that applies a disease on the target in addition to base damage while Blood Strike deals damage based on the number of diseases on the target. Death Knights are also able to raise Ghouls to fight for them and are projected to have an ability that applies a disease on their targets. Load up on Airborne, because it's going to get pretty toxic in Northrend.
One can only speculate what school of Magic the diseases will be based on. It's possible that they'll be Shadow-based considering the nature of the class but the probable culprit will be Nature, as virtually all disease debuffs in the game are currently Nature-based. The possibilities are exciting, but for now it's good to remember what abilities are out there to combat diseases. There are currently three classes that can remove diseases -- Paladins, Priests, and Shamans.
Paladins can Purify and Cleanse, while Priests can Abolish and Cure Disease. Shamans can also Cure Diseases as well as drop a Disease Cleansing Totem for AoE curing. The Dwarven racial ability Stoneform can also remove diseases, giving Dwarfs additional solutions against Death Knights. A couple of Alchemy concoctions, Restorative Potion and Purification Potion, also combat diseases but are suspect because of their low item level. Given that Death Knight encounters will be high level, these items are likely to fail.
Wrath of the Lich King may be a long ways off (we'll find out more during Blizzcon, I suppose), but it's exciting to see that Blizzard is exploring new ways to expand the game. With the advent of diseases -- and hopefully we'll see more Death Knight abilities based on them -- we'll see the use of abilities that sometimes don't even find their way to some players' Action Bars. Besides, it never hurts to prepare how to counter a Death Knight... from the looks of it, we'll be seeing them everywhere.