Plague & refugees
Two population forces move people across the map: a global pandemic that spreads country-to-country like an SIR epidemic, and steady economic migration flowing from poor nations toward rich, stable, open ones.
How it works
Plagues ignite randomly — roughly a 1% chance per tick with no active outbreak — seeding one country at severity 0.3. Infected nations lose population and GDP each tick scaled by severity, with stability dropping too; the damage is cut by that country's containment, which comes from health-budget spending, closed borders and lockdowns, capped at 0.85. The disease spreads along trade links and geographic proximity, amplified by open immigration policy. After it reaches 16+ countries (or 30 ticks pass) it peaks and begins to decline, and recovered countries gain immunity so they can't be reinfected — many outbreaks fizzle into mere regional epidemics.
Migration is a globally conserved flow: it only moves people, never creates them. Every country gets a push score (rising with poverty, low stability, civil war and being at war) and a pull score (rising with per-capita wealth, stability and open immigration policy, cut by isolationism and war). People only ever migrate from poorer to richer nations — the sim sorts by per-capita GDP and lets each country absorb from the pool of everyone poorer than it.
Absorption is deliberately capped so small nations can't balloon: ordinary countries have a tiny cap, but a deliberate magnet (open-migration focus, magnet decision or investment-immigration) unlocks a much higher ceiling up to 0.034. Arrivals dilute per-capita GDP on a 7:3 basis (destination 70%, origin 30%), can raise a language-minority share, and cost some social cohesion (stability) — while investment migrants instead lift GDP by 1.5× your per-capita, modeling wealthy inflows.
Formulas
pop *= (1 − sev*0.011*(1−cont*0.6)); gdp *= (1 − sev*0.018*(1−cont*0.5))cont = clamp(health*0.012 + closedBorders*0.25 + lockdown*0.4, 0, 0.85)pull = clamp(perCap/15, 0, 2.6) * (0.55 + stability/100) * policyFactorKey numbers
How to play
- ▸Fund health budget and — in a crisis — close borders or lock down to push containment toward its 0.85 ceiling and blunt plague deaths.
- ▸Recovery grants immunity, so riding out an outbreak once can be safer than repeatedly reopening; survivors won't be reinfected.
- ▸To grow via migration you must be a deliberate magnet: an open-immigration focus or magnet decision unlocks the high absorption cap.
- ▸Mass ordinary migration dilutes your per-capita GDP and costs stability — use investment-immigration if you want inflows that raise wealth instead.