Medicum Morbo Adhibere

 I've been having a lot of conversations lately about BX/lotfp-compatible disease mechanics, and it seems like there's not a lot of resources out there for something more complex than 'PC picks up disease, suffers from it, maybe dies, maybe passes it on to their party members'. Me being the person that I am, I'm kinda obsessed with disease - the spread of disease, the effects of disease on a population, etc. I mostly blame Ice Pick Lodge's absolutely genius plague-based horror survival sim Pathologic, but the ongoing global pandemic has kinda... intensified my fixation a bit. 

So I've been running a campaign since last September where Eastern Europe is being ravaged by the Red Death. Like from the Edgar Allan Poe story and Zzarchov Kowalski's excellent module The Punchline. And, given that there doesn't seem to be a lot of this kind of thing out there, I thought I'd share the disease/pandemic mechanics I've been using. These mechanics assume something very catching and very deadly. I'm using the Red Death, but this would also work for something like the bubonic plague, Spanish influenza, a zombie virus, the Sand Pest from Pathologic, or hell, COVID-19. The emphasis is on how the situation effects the PCs, what steps they can take to potentially improve the situation, and what will be lost as the situation gets worse and worse (as it undoubtedly will).

The Red Death
The incubation period for the Red Death is three days. No more, no less. 72 hours exactly. When that time is up, you're dead in less than half an hour. 

When a PC is exposed to the Red Death—usually through coming in contact with infected blood—save vs. Poison. On a successful save, you're safe; you got extremely lucky this time. On a failed save, save vs. Poison again after the incubation period. Fail, and your Constitution score is lowered by 5d6 (3d6 on a successful save). 1s and 6s become permanent Constitution loss. If your Constitution reaches 0 or lower, death is instant.
The recovery process for a survivor is slow; you regain 1d3 lost Constitution per week (1d6 with bed rest and adequate nourishment in a clean room). This healing process is long and painful, but you're in luck: if you survive the Red Death, you're immune but still able to carry it.

1% of the population is immune but can carry the disease. This chance is rolled at character generation and kept secret.

NPC Infection

So you don't have to track all kinds of annoying fiddly things, infection for NPCs is simplified. Each fortnight, roll d% for each named NPC. NPCs have a base chance of 60% to avoid the plague, +/- 10% for each point of Constitution modifier (positive Constitution modifiers equal bonuses, negative Constitution modifiers equal penalties). -10% to the roll if they're infected with another disease or afflicted with another sickness, if malnourished, injured, or in intimate contact with someone infected (if you aren't immediately sure if they've had intimate contact, roll 1d6. 1-3: yes, 4-6: no). -5% per Infection Phase (past the first) of the settlement they live in. +10% if they're in the prime of their life or if they isolate. +20% if they have a doctor living in their home. Maximum 90%, minimum 10%. If they fail that roll, save vs. Poison or they become infected.

Settlement Infection
Each fortnight, make a 1d20 Resistance roll for each infected settlement. Roll above 1 + (1 per every 500 people living in the settlement) + (current infection phase of the highest-phase nearby settlement) + (the settlement's current phase).
The difficulty of the Resistance roll is 5 points lower if a settlement has proper sanitation, and 1 lower for every 10 doctors living in the settlement (assume an average of 1 doctor for every 300 people). On a failed roll, the disease spreads to adjoining hexes and locations connected by railways and major roads. When a settlement is infected, the disease progresses in phases on failed rolls.

Phase 1: 1-in-1000 people you meet are infected (These numbers are to keep things easy for the GM, not to accurately model the percentage of the population that's infected). If the plague is contained now, 10% of the population will die. Food costs are x2.

Phase 2: 1-in-500 people you meet are infected. If the plague is contained now, 20% of the population will die. Food costs are x5.
Phase 3: 1-in-100 people you meet are infected. If the plague is contained now, 30% of the population will die. Food costs are x10.

Phase 4: 1-in-20 people you meet are infected. If the plague is contained now, 60% of the population will die. Food costs are x25. Incoming trains shut down, outgoing trains are at x100 cost. (I personally have trains in my setting because I think they're fun, but if you don't, replace this with some other type of outgoing transportation that PCs will have to pay for).
Phase 5: 1-in-6 people you meet are infected. If the plague is contained now, 75% of the population will die. Food costs are x50.
Phase 6: 3-in-6 people you meet are infected. If the plague is contained now, 90% of the population will die. Food costs are x100.

3d6 Plague Encounters
3: A mass burning of corpses.
4: A house being boarded up by the military police.
5: A cart full of bloody corpses, the cartman shot dead in an alley.
6: 2d6 plague doctors make their rounds.
7: A man sells a miracle cure. It's actually just bottled piss.
8: A funeral procession for an influential noble.
9: 2d4 escaped madmen.
10: An infected witch on the run, followed by 2d6 soldiers.
11: Someone dies at your feet.
12: A looter is shot in the head by a soldier.
13: 2d4 concerned family members try to break a house's quarantine. They will tear down the boards with their bare hands.
14: The gutters run red with blood.
15: 1d4 infected and dying try to wash the blood away in a fountain.
16: A band of 1d6 infected seek someone to kill to indulge a superstition that one can escape the plague through killing.
17: A maddened infected noble tosses coins and other valuables out their window.
18: A new law establishes quarantine regulations. Doubtlessly, they will be ignored.