Interesting to have different opinions. Maybe you're right and Watashiwa and I are wrong about the need of further softening disease.
Maybe a good way to solve the diseasplosion would be to let the balance as it is but solve the "control" feeling. If TheVampire says it's fine both ways, after all, why not ceasing hammering this poor disease and letting it be a little nasty, after all. Maybe, just displaying somewhere how does the diseases strikes and at which threshold. I guess there is a computation involving pollution and population behind the hood; displaying it as a manageable threshold would make the player "in control". A situation that I can think would be good for this control problem would be:
I have a few hazmats and research labs and my pollution is maintained below the disease threshold; if I want more labs/military/industry, I need to prepare the ground for disease (researches, graveyards, trauma centers, etc), makeup my city, and endure the disease.
or
Very early game is behind me, now I want to get all the economy/military/science that lies behind the disease wall; am I ready to trigger the pollution switch and cross the disease threshold?