Disease, by its nature, will tend to make the game less fun for no real benefit.
The only potential benefit I can see in disease is using it to force people to split up their armies more or pay more for extra precautions. But that just doesn't make up for the drawbacks involved.
i am aware that discussion is already elaborated much before, but when you are mentioning less fun, could the same apply for wounding/imprisoning/paying guildhouse costs and some other things which never bring anything positive?
clearly, nobody would like that his characterd dies all of sudden, but if non-hero nobles are immune from battle deaths, could they be immune to decese deaths too?
just a though - random decease hits some region-two now and than, halving population, degrading economy, and putting nobles who appear to be in a region at time of occurrence in critically wounded state for few days.
this will certainly not be fun for region lords and their knights, but if limited to few regions, such events could disturb some hard-lock peaces or power balances to some small extent, that would just allow some rp, and shake things a little.
some neighbor could use situation to attack weakened stronghold, or someone would try to corrupt local government, or war balance would be randomly disturbed to some small extent etc.
