Maybe some of these come about due to curses being inflicted, using too much magic in one go and so on, but you could explore what diseases would exist on your world. Then there would be stories in how such diseases are treated, what are the equivalent of doctors and nurses in your fictional setting, what would be conventional and alternative medicine and how people react to these sicknesses. (Is your fictional world a compassionate one or not?)
Could sickness and disease be imported from worlds your main world has contacts with and how does the populace react to this? (It could easily lead to racism, a wish to become isolationist etc). If your world is, somehow, sickness and disease free (really? No bacteria anywhere then?), how does your society maintain that? What bodies do your main characters have - are they like human ones with a need to have friendly bacteria resident inside or are there systems like an exoskeleton type and perhaps when illness comes in, that's it, no treatment possible.
How do people/magical beings etc react to when they are ill/loved ones are ill? A lot of character can be revealed in reactions.