Undead and Ctis Miasma

[Action]

Using a Prince of Death with Ctis Miasma.

Under the description of the undead status it says "Undead are not affected by disease".

However, the disease affliction just popped on my PoD while he was sitting in the capital.

Is this intended?
Will the disease give him afllictions or prevent his natural healing?
Can he spread the disease to my cold blooded troops?

Thanks.