That could work, but people don't enjoy having limited options I've found. Maybe more classes could work like the cleric, the Artificer (specifically the Alchemist), and someone pointed out to me that doctors still need guards, and maybe Barbarians/Fighters could be that.
Yes, I agree, just fighting the disease would be lame, and maybe one of the reasons the disease is so deadly is that magic doesn't work on it? What would you recommend said special rules would be?
It would most likely be a horror campaign. I was thinking along the lines they are trying to treat a magical disease, but at first they would think they are treating a normal disease.