I love the idea. We allready have herbs, sophisticated health system, why not diseases? IMHO random contraction wouldn't be a great idea, diseases don't come out of nowhere, but most diseases come with certain options:
- You eat bad food - You get Yourself poisoned;
- You get pierced with an arrow and forget to clean your wound - You get blood infection;
- You eat rotten meat - diarrhea;
Just imagine how siege tactics would look - besiegers throwing dead cows with trebuchets over castle walls to cause water poisoning and spread diseases. Amazing
P.S. Critical health conditions hypothermia should be included as well (damaging stamina/health bar etc.) - so no swims during winter