How to Defend

[Vynd]

and because mr - negates at least most of your mages / scs are normally not diseased only parts of your army . but they are fodder anyway so don't care and you only lose 1 hp / turn when diseased . so when you have tough units ( e.g. vine ogres ) it takes a really long time until they die ( about 50 turns ) from the disease .

Well this is making quite a few assumptions. For one, that he's relying mostly on Vine Ogres or similiarly beefy troops. Disease is not so trivial if you have an army of, say, Vans.

Also, Disease causes additional afflictions, so even if you are using big beefy stuff like Vine Ogres they will probably end up blind, crippled, and useless long before they actually run out of hitpoints.

Assuming you do not have easy access to healers or Gift of Health, I think the "find and kill the casters" approach is best. And if you are on the defensive then this is just one of the many reasons that you would want to put some Dome spells on provinces where you keep large armies. Other than that I don't think there is a good way to defend yourself against stuff like this.