Simple change, make it so that Plague can be spread by Undeads on offensive operations, but will not be spread when they are attacked by others, make it an ACTIVE thing only.
Right now it's extremely hard to fight UD KDs as your TMs pretty much cannot take land from them once you put them on landdef. Chance of getting plague in a quad-tap are about 100%, and so far this age my experience has been that NB doesn't have a cure rate of 20% but more something inbetween 10-15%.