Pestilence-

As a swift action, a Death Knight of 3rd level or higher may spread diseases from one target to all nearby foes. When using this ability, the Death Knight selects one target within 30 feet; all diseases on that target are transfered to all enemies within 10 feet of the target, with the same duration as the original. Diseases transfered in this way deal 50% normal damage.
The wording on this is a bit...iffy. "Spread" implies that the diseases end up on both the original target and the newly hit ones, while "transferred" could imply that the diseases end up being removed from the original target and only affect the newly hit ones. Could clean it up, or just a "Diseases are not removed from the original target" line could remove any ambiguities I read into it.

I know how it's meant to work of course, just that the language isn't perfectly clear.

And yeah, I think the anti-multiclassing clause might be a good idea.