[thejeff]

Except that you can't put them on the mages, because they get diseased and die.
You can put them on Black Servants to ferry gems along with the mages, but you still have to distribute them every turn, just as you would if you'd just shipped out gens on normal scouts. (Which you have to do anyway for non fire gems.)

I suppose it's still useful in some situations.

The description makes it sound like it should only work on those who actually suffer from disease. I always assumed it working on undead was something of an exploit. Might be the intent though.