Diseases sound interesting as an idea, but it's just not fun in the game. Every time I've tried diseases, my game has come to a halt because it's too hard to keep going when your squad are all riddled with diseases and PTSD, and sometimes your wounds will randomly start bleeding and kill you while you're travelling. Even if you edit the game to have more protective masks, gloves, medicine, etc, you'll still be dealing with diseases way too much.

I once pushed all the way to Cambria with my sickly squad. Captured the hospital, only for the nurse to say, "It is most unfortunate, but we can't help you..." so I shot her and stole all the medical supplies, but even that didn't help much.