Leprosy was a very powerful diplomacy tool in an MP game I played recently.
I was playing as Pangaea and Tien Chi invaded my lands. As they invaded my lands, Atlantis followed behind and gobbled up the Tien Chi lands and ventured into mine. In attempt to hold some of my former provinces, Atlantis massed quite a few troops in my former provinces even though we were at peace with one another.
I casted leprosy and on the same turn followed it with a message telling Atlantis to leave my rightful territories. To my surprise it worked pretty effectively. If Atlantis would have pushed the issue, I would have been hard pressed to stand against him after recovering from the war with Tien Chi.
So in that instance Leprosy worked pretty well as a message short of war.
