The fever, and how it gets about
A fever is caught rather than rolled: it passes between people in the same place in the same hour, harder under a roof and harder again in a crowded one. Strangers carry it up the road, and a reeve can shut the square against it.
It is caught, not rolled
The old rule was a flat chance an hour against nothing. Nobody caught anything from anybody, which is the one thing about illness in a village of four hundred worth simulating at all.
Now it passes between people who share a place for an hour, and where they are decides how much: a house is worse than a lane, and a crowded house is worse again. What is left of the flat roll is bad water from a fouled well, which genuinely does come out of nowhere.
The tenement cuts both ways
A six-door tenement is how a village houses forty families, and it is also the best thing that ever happened to a fever. The room built to solve one problem is the room that gives it to six families in a night.
That is the point of building it into the housing rules rather than beside them. A reeve who builds up gets a housed village and a more dangerous one, and neither half is a dial you can turn without the other.
It comes up the road
Strangers are the only thing that enters the valley from outside, and how many come is renown, recomputed daily from bread, ale, mood and hunger. A village worth visiting gets visited by whatever is going round, so a good year is what buys a bad one.
What a village can do, and what it costs
Everybody with somebody ill under their roof can sit up with them, which helps them and is how you catch it. Anybody can shut their own door and go nowhere. A reeve with enough of the village down can shut the square, take a barn for the sick and pay two to sit with them, or shut the bridge.
None of it is free. The square is the only thing in the valley that turns strangers into acquaintances, so a month of quarantine buys a healthier, lonelier village, and a shut square is shut to the market and the fair as well. The rules refuse a reeve who wants to shut a square over one sick child.
Measured, not guessed
Over five years on three seeds: eight to eleven in bed on an ordinary day across both villages, a bad visitation peaking at twenty-odd, one or two of them named, and twenty-seven to fifty-three buried with it. Set NO_PLAGUE=1 to run the valley without it and read the difference, which is how those numbers were got.
The first draft killed a hundred and sixty in the same span and had twenty-eight people in bed every day of five years. Nobody works from a sick bed, so it starved the valley as well: hunger deaths went from twenty-five to eighty-seven. Every number it runs on is a lever in one block at the top of plague.ts.
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Land, building, and a house that grows
The village sells parcels along its lanes, and the ones nearer the green cost more. A family that buys one marks out a site the yard raises, walls first and roof at thirty timber and five stone, from hovel to house to second storey to manor.
A closed medieval economy, in real time
Two villages of about 220 people each run a complete production chain, field to mill to bakehouse and pasture to butcher, with floating market prices, daily wages, household purses, tax, tithe, and a winter every twelve days.
The crowd's hand
Every ninety seconds watchers are offered three things that could happen next: a fire at a named house, twins for a woman who is carrying, wolves, damp in the grain, soft rain, a pedlar, a feast. The one they pick happens.