Death Knights. Seems like they're all the rage these days, and they haven't even been released yet. It was revealed that Death Knights start at Level 55, the only requirement being that the player has an existing Level 55 character. Daniel has even declared his intention to make a Death Knight his main when the time comes. I wouldn't be surprised to see a proliferation of Death Knights when Wrath of the Lich King is finally released. When that time comes, prepare to get inoculated because the class is the first to finally use diseases as a core class mechanic.
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.