At the moment, I have no rules for players catching a disease from out of the blue; i.e., the player trips and falls face first into a mud puddle after leaving a tavern, wipes himself off, leaves a bit of residue from the dwarf that peed there an hour ago, feels bad the next morning, finds the disease in full swing by afternoon. No rules. My players don't mind.
If I were going to create rules, I'd probably start with some ballpark estimates for medieval death rates. Here are some English numbers from John Hatcher's Mortality in the Fifteenth Century: Some New Evidence:
Those are quite unhealthy. Apparently, it was hard to be a Cleric; harder, we suppose, to be the common population, but then characters aren't common. For the record, tenants-in-chief refers to whomever happens to hold the land for a fief - typically a captal or a baron. The monks at the priory of Christ Church were Benedictine monks, the classic cloistered monks who set the standard for a cloistered existence.
So the columns above describe two groups of relatively active but home-bound persons, at the top of the food chain . . . and still we're talking between 165 and 224 deaths per thousand per year for a fellow my age. That's better than 1 in 6 that I'll be dead by next summer.
For comparison, the death rate for Canada in 2013 was 7. The death rate for Afghanistan that same year was 8. The highest death rate for anywhere in the world was Sierra Leone, with 17. If any country in the world right now had a death rate as high as the above table, the world press would be freaking out in ways that would make Rwanda look like a passive affair.
Still, we must assume the death rates for the artisan and peasant classes were higher. Not that everyone died of disease. Executions for crime, accidents, deaths from war and pillage, all these things were higher. There was a much greater chance of being killed in the street because of a riot in those days. So we have to restrain ourselves from arguing that a 20-year-old fighter has a 6-10% chance of catching a disease this year.
However, if we consider mortality rates from disease as a guideline, we see that Canada's morality rate from all diseases in 2009 was 2.2 per 1,000 (220 per 100,000). That's 2/7th of our total death rate . . . in a world where most of the horror-show common diseases from the Middle Ages have been successfully conquered. We're still losing 31% of our people to disease.
So, arguably, we could double that ratio to 4.4 over 7, then multiply that against death rates for the youngest tenant-in-chiefs for the latest period in the table above. That gives us a ballpark figure of 64 persons per 1000 population dying of disease each year (6.4%). That's about 1 chance in 569 per day.
Whereas I could simply make a table in excel that would roll up that chance at the push of a button, I haven't felt the necessary desire. Players are not enamoured with dying from disease. They're cool with catching a disease from giant rats and other things, that's all part of the combat world. But catching a disease from a mud puddle? "No fair, man!"
It is a very hard thing to argue against, however, from a player's perspective. We can't argue that disease didn't exist; we can't argue that spending weeks in dank dungeons, slogging through storms to and from town, camping outdoors and spending most days covered with welts, cuts and occasional deep gashes isn't a breeding ground for little buggies. But . . . if the paladin in the party is just going to heal these regularly as they come up, is there really a point in keeping track?
Well, the paladin could get overwhelmed. A cleric with a cure disease spell, too. And the problem gets bigger as the parties do. I just had a thief in my party hit 10th level and pick up 19 henchmen. That's a LOT more chances for one of them to catch a disease and die while the paladin is off gallivanting in a nearby dungeon. Shame to lose a 4th level follower that way.
For the present, however, I only plan to tweak the old DMG's disease formula - which I find isn't bad, if lacking a little on the details. I'll be working on this for the wiki between humping my ass around trying to find a cooking job.
I wrote this post more or less on the spur of the moment, inspired by the table I stumbled across above. In it, I've realized now that an hour has passed that I spent no time whatsoever considering the contraction of disease that does not kill, but continues in a more or less chronic matter. Nor had I given any thought to mild cases of illness that, although they wouldn't kill the infected, would seriously affect the infected's ability to participate in most adventures.
As such, clearly, this entire post needs a rethink.