So, let's do some simple design, just to get a handle on some new rules for disease. Disease is one of those things in the game that everyone hates, players and DMs alike, because it is so random, so out of the blue and in turn so deadly, it seems grossly unfair to arrest a player in his or her tracks just because the percentile dice comes up '01.'
Looking at the vegetation map I posted for Dalmatian Lands yesterday (the second map down the page, mostly green and yellow), the gentle reader can see numbers that are black on white. These are degrees of civilization, or what I was calling 'infrastructure' nine days ago.
(See? There is method to my madness. First, I introduce the math, then the data, and finally the purpose)
Let's take a wide variety of places. We can pluck them all from southern Italy. Hopefully, the reader can find Naples ... its in the middle of a bright yellow area in the lower left quadrant of the map.
Naples has an infrastructure (IS) of '3478' ... which is the highest number I could find anywhere that I've actually calculated. Paris might be higher, but I haven't actually calculated it. The number is based on the relative density, and in the middle ages, the density around Naples was HIGH. It was, at that time, one of the most urban places on earth.
Above and to the right is a hex with Benevento, with an IS of '456.' Two hexes to the upper right, the reader can see Lucera, with an IS of '88.' One hex down and to the right of Lucera is an empty, dark green hex with an IS of '30.' And up and to the right of Lucera is a empty, light green hex with an IS of '5.'
Let me blow up the relevant area so it's convenient. The places I've described are makred with blue arrows.
Now, it can be seen that there is another small number in the right corner of every hex, in black. In Naples, that number is 0. In Benevento, it is 275. In Lucera, it is 610. Finally, in the dark green hex it is 2814 and in the light yellow hex (near San Severo) is 2109. These are elevations, in feet. On the original map, the reader can see that these elevations (and the low IS numbers accompanying them) follow the Apennine Mountains down the spine of Italy.
Let's take advantage of these numbers being based on density and lets say that the number itself, divided by 10,000, gives the % chance for becoming diseased if you spend a night in that hex.
WHOA! A 35% chance for disease if you sleep in Naples? Ridiculous! The population would all be dead!
Yes, that is probably correct, except for a few points I'd like to make.
First, I said becoming diseased, not catching a disease. I'm arguing that you will contract something that first night ... but not that there is then again a 35% chance of catching something completely different the next night. What I'm saying is that if you lie down in a gutter for three nights in Naples, then yes, by the end of the third night, you will probably be sick with something.
Second, I must rush to point out that in all probability, most persons in the medieval world probably were diseased ... with things we hardly think about: a wide variety of skin maladies, fungus, moderate pneumonia or tuberculosis, not to mention tape worms and other internal beasties, scabies, mild forms of disentary, scurvy and a host of vitamin deficiencies, etc. All that and chronic malaria, for another thing, which was rife in 16th century southern Italy, particularly among the coastal urban population.
Finally, this is D&D ... so the reader should expect modifiers. 34.78% is just a base figure.
Note, however, that as one gets further and further from the city, climbing up into the mountains, the character actually becomes safer. Don't want to die of disease? Don't go to town.
Paladins, naturally, are immune from disease anyway, and they could keep the diseases of a small party fairly at bay. But a lot of time spent in Naples could make cure disease as useful as cure light wounds.
Features that would prove useful combatants against disease would include public baths, the prevalence of falconers and rat catchers, wells for fresh water (if it was fresh - wells helped kill people of cholera in 19th century London), hospitals, apothecaries and gong pits (which would presume a steady effort to clean up). Dug sewers, too, would be a feature which I hadn't mentioned before.
This is Naples, however, with a population exceeding half a million and surrounded by a million more in the surrounding cities and duchies. So it can't be a question of whether or not Naples has sewers, gong farmers, a hospital and so on. Obviously, it would have many hospitals, and lots of everything else besides.
What matters isn't the presence of these things, but the degree of presence. And here we get to pull away from the flat numbers. Has the town kept up the sewers? Are the hospitals intelligently run, or are they warehouses for the dying? Are there enough apothecaries, gong farmers and rat catchers? Is the water in the wells very fresh or awfully tainted? What standards apply to the baths, and are they in the best of maintenance?
Suppose we make a supposition that each feature (seven in total, baths, ratters, wells, hospitals, apothecaries, farmers and sewers) has the potential for reducing disease in that area by 18%. Thus, the potential could be to eliminate disease in Naples. More to the point, however, 3d6 are rolled for each feature to see what is effective, and how much. The average says disease will be lowered, in toto, 73.5% ... but a couple of bad rolls could make Naples into a disastrous vacation choice.
The same rolls are applied to every hex ... that is, that has a town (please note that the danger of disease applies to staying anywhere in the given hex, not just the town itself - though you could double the number for the city and halve it for the country, if you like). Benevento and Lucera are a lot smaller, so they have less of everything ... but they also have less disease to combat (Benevento a base of under 5% and Lucera with a base of under 1%). All in all, both could be quite decently safe.
The hills, of course, could not have sewers or baths ... but 'hospital' is a fairly wide term and in any case, there could be a local physician, along with a local herbalist and rat catcher. Overall, there would be slightly less reduction ... but there's also a lot less disease to reduce.
Now, once the party is informed of the principle, they would know what chances they were taking ... by actually assessing the local towns. They'd be asking after hospitals. They'd be tasting the water upon arrival. "Is there any evidence of sewers?" "Have I even seen a rat catcher today, or someone with a falcon?" And so on. Bang. A town now is more than just a collection of houses. The players care what the infrastructure is.
More to the point, it would be REALLY HARD to raise Naples' sewer system from your ... call it a 'mitigation' roll ... of 7% reduction to an 8% reduction. That's a lot of time, labor and money, where it comes to rebuilding parts of disease-infested Naples. But improving Lucera by 5% might be something a mid-level party could manage - now that there is some logic to what needs to be done and how it is to be done. The town has one crappy hospital - build another. The town's baths are basically three natural ponds. Let's shore them up and make the townspeople appreciate us. "Let's spend our money on something concrete, as opposed to just more armor!" And let's have some adventures grow out of the struggle to overcome the miserable process of living ... when there are people out there to oppose or help our efforts.
You want an adventure? There's an adventure.