I agree with what LadyWyT says and would like to say that in my experience, diseases in games are generally not fun...
Unless you make them lesser than in real life. I thought Skyrims disease system were quite good, rarely were affected by it but when you were you were like "cool, now I have to visit a town or drink this potion because I got bitten by a Wolf which had a disease". It made the world feel more alive in a sense but just enough not to burden your tasks you were committed to do.
So having to make a herbal tea and maybe go to bed early and having the effect lessen by how long you sleep good be a cozy mechanic without burdening the player too much. In real medieval times you would not do much if you got sick for weeks.
Diseases caused by animals could maybe be blocked by using a shield so they dont bite your arm, or atleast lower the chance of getting bit... Something like that would be cool.