An example Brian gave was how anticlimactic it would be if the real dangers of the world turned out to be what actually kills your adventurer.
John: Yea…too bad after all that you drank the castles well water, caught dysentery and died after several days of severe diarrhea….
My question to you is this – would you like to see a threat of disease? Would there be any fun to it?
My initial reaction is, unless it’s linked to a specific plot device – no, there’s not really any fun to it. On some level, we will always have the conceit that we’re playing a heroic fantasy game – even if it is a gritty one. You don’t see half of Westeros dying of parasites or polio or something. Heck, in Game of Thrones, if someone does contract a “disease” it almost certainly poison (there are some exceptions, but even the exceptions are plot devices – but I’ll keep this spoiler free).
I think it’s worth writing some rules for disease – let some threats (Mummies) or known areas (The Plague Bog) carry a threat of disease – but even then, if you decide to brave the plague-infested town to save some lives, and you come down with the plague and die, are you going to enjoy that session? While realistic, it seems unfulfilling.