I'm pretty new to this, so my opinion isn't really worth much, but just looking at it as a map, it occurs to me that (1) mountain chains don't exist as a single row of peaks, they normally have more width, and (2) in addition to mountains, rivers are a barrier to the spread of anything, so you might want to add rivers to help delimit the infected areas from the non-infected ones.

Also, the rings are all relative to a point source (town or whatever). The full overall spread of the plague could be done with faint color on the large white expanses, to show how it spread from town to town.

Just some ideas. Overall, I like the map and what you have done with it.