I think the OP means this:

Gather 40 mobs on you, and as soon as you put a disease on a mob it dies, so no way to spread it. With a DK you are limited to your runes & weapon swing speed on how fast you can kill. Thus it's way slower than what consecrate would do for example. Scarlet monastery: I don't run towards each mob, but I usually just pull the boss or run within aggro range, to pull large packs. However .. how about death & decay, you can even glyph it just for your boosts.

Another idea might be to go for a boss, put the diseases on him. He might stay long enough alive to actually spread it