[Edi]

NTJedi, I know very well what diseases are. Most you either recover from or don't. I'm assuming that the diseases in Dominions would be, like Kristoffer said, of the invariably fatal variety (such as leprosy) or tuberculosis or syphilis which the victim will not recover from without medicines/magic and which will be fatal over the course of time.

As for smallpox and plague (bubonic, lung and blood), while those were not always fatal (not sure about the blood variety, which was worst, they're all caused by the same germ), they were so deadly that they might as well have been, and it's just an added layer of complexity to model some minuscule recovery chance and attendant later immunity to disease for such recovered units into the game. The Black Death basically depopulated Europe in the Middle Ages, so I don't think it is an unrealistic simplification, or even a particularly far-fetched one, to have units that catch disease die eventually.

Edi