The disgust that attaches to disease-carrying substances or people is known as basic disgust or core disgust. It's clear that people in earlier technological ages had a much higher threshold for this than we do now. In medieval European cities, waste disposal was done in the street. Late-medieval advice on courtly manners seem to be made for a modern five-year-old, telling adults not to wipe their nose with their sleeve.
|"Dennis, there's some lovely filth down here."|
All of this misses a huge point: disgust is a language. It needs to be translated to produce the right emotional effect against the cultural background of the audience. In just the same way David Milch, writer of the HBO series "Deadwood," recognized that his characters would have historically sworn by God, Christ and Hell. But because those oaths sound mild to the ears of a more secular 21st century, he intentionally replaced them with the Tarantinoesque obscenities that series is famous for.
The effect of putting a crap bucket in every kobold lair in gaming, too, is rarely as intended. It drives a wedge between the modern sensibilities of the players and the medieval disgust thresholds of the characters. This fights against absorbing play, which depends on making the players feel what the characters might be feeling. Worse yet, in gaming you are only depicting or talking about disgusting things (thankfully). This can lead to another unwanted response to second-hand gross-outs - laughter.
To be truly effective, disease-related gross-outs in a game should be few and far between. They should have the rules back-up to make both players and characters recoil. The rot grub in AD&D has been much maligned. But knowing it is out there did make players a lot less willing to root around in dung piles. It visibly stands for the invisible diseases that disgust at its most basic level protects us against. Rats, mummies, anything foul that gives a chance to catch a disease, likewise add to the keep-away factor.
|Try grubby instead of gritty ...|
Rolling chances to catch a disease for living in a city have never been satisfactory in gaming, even if they are historically accurate. The city is just too attractive a destination for your adventurer's purposes. Its disease perils are too abstract, failing to arouse burnt-out medieval sensibilities about squalor. But ordering disease rolls for too-cheap lodging, or hit point losses for sleeping rough, gives players something they can avoid, giving meaning to their fastidiousness.
There's one other thing about disgusting objects that most old-school rules don't model: they're contagious, like those invisible diseases. Once you have players figure out that someone with a disease has a chance of transmitting it to anyone nearby, you bet that curing them and setting a quarantine will take on urgency. A green slime that kills you instantly is bad enough, but one that settles on you and keeps you alive just long enough to send spores out to land on your friends is worse, a gruesome kind of living death.
Next up: Mutations and mutilations!