If this is based in Shadow World, you should consider a few things, some have been hinted at.
The features of the desiease could be mapped out like any other opponent. What its nature, how it spreads, can it re-occur? How fast does it mutate? Once mutated, are previous victims immune or not? How long does it take to show symptoms. What are the phases. When is it fatal? Does it impact the young and old or all or perhaps only fatal to those with a certain racial inheratince, etc. how about are there a set of people immune but are carriers? Do animals or insects spread it?
is magic a large part of your game of Shadow World. Thus disease itself may have magical elements. I would suggest this is represented not as a level but as a base % resistance to being forcibly cured by any spell, potion etc. even a small percent of 5% - at which point the spell to be tried again must be done by a more powerful caster, item etc, would still statistically impact a large portion of the population.
In a world like Shadow World, any moderate successful government knows it needs a plan. Especially considering how many different disasters are waiting for the unprepared. Especially the Sky City and others. They will have plans to deal with an epedimic. Need to figure out how it works and what it is.
Last I always feel that game spell systems are long on combat n action spells and totally ignore that in a magic rich world, other day to day and craft magic will exist. Spells in Spell law are the magic of the combat casters. There should be more. I dislike a city with 1% of the population being involved in religion and all of them being the class cleric. Does not make sense. Of course it takes a bit of work. It also answers questions of how a smaller farmer population can feed a larger cities, in a magical world. It also opens up possibilities.
Imposing our medical history on a world and not changing things, as technology changed things for everyone misses opportunity.
Sorry for any off topic
Cheers