I don't see disease being fun in a Mount and Blade setting. Characters are not disposable, lords have personality, and well-trained soldiers are expensive enough as it is (not just financially, but in time too.)
Characters are not disposable: As the developers have stated, your character has the possibility of dying. With that being said, I would like it if there were zero random factors pressuring the death of my character. Although there is the possibility of heirs in Bannerlord, I fear that I would despise the idea of randomly dying and having a new character take my place. The game is all about building your character's skills and relations.
Lords have personality: Let's say there are two lords. Lord A being very honorable and prudent, always supporting the morally right choices and treating his fellow lords with respect. Lord B is very independently minded and doesn't care so much for what other lords think. He finds it foolish to allow lords free after fighting a fair fight in the name of honor and respect. If we were to add the disease mechanic, then it would be pointless for me to describe these lords at all. They would likely die after several hours of gameplay, meaning that learning their personality and their name would be completely pointless and take away the liveliness of the game. Perhaps one lord is very bloodthirsty and kills other lords upon imprisoning them. Although that kills off the personality and life of lords, it means that it adds character to another lord. By this lord killing another lord, it gives the player emotion and thinking towards this lord. disease would take away from any player attachment to lords.
Well-trained soldiers are expensive enough as it is: Imagine sieging a castle of 140 soldiers. You have 120. It's likely that you will lose, but you have conquered castles like this in Warband. Disease wipes out much of your Vlandian Sergeants and Crossbowmen. You now have 95 soldiers and the enemy keeps their 140 strong castle. Not only did you fail to conquer the castle, but you have lost many soldiers that have taken valuable time to train. That is not fun and not quality gameplay. It's different to lose soldiers auto-resolving or simply fighting on the field. Fearing the chance to lose soldiers to disease is not something players should consider when sieging or just traveling on the map.