Ok, as I mentioned above I was playing a MA Agartha game. I had Growth +1, and my only land opponent (after knocking of Machaka) is Jotunheim, with Death 3.
To date, around 3 - 4 years (I need to check), I have had exactly 2 mages contract disease in my dominion, and 2 others get exactly 1 wound each (neither was disease or feeblemindedness). That was out of at least 16 mages.
I have also sent an army of ~10 mages through Jotunheim's death 3 dominon in the war. They all died in a large attack from Jotunheim, but prior to that 4 got diseased, another 2 got wounds, and none were feeble minded.
So, to recap, I lost 5 to disease (1 got feeblemindedness at the same time, so I think that counts more), 1 to feeblemindedness, and 4 got non-debilitating wounds. Keeping in mind that I have another 10+ turns of use after a mage gets diseased, I don't think it is that bad at all.
It could be that growth scale help. I know death scale really hurts.
