Stochastic SIR Epidemic
Agent-based infection spreading on a grid — click to seed
About: The SIR model (Kermack-McKendrick 1927) divides a population into Susceptible, Infected, and Recovered compartments. Each infected cell infects each susceptible neighbor with probability β per step; infected cells recover with probability γ. The basic reproduction number R₀ = β·neighbors/γ; epidemics occur when R₀ > 1. This stochastic spatial version shows heterogeneous wave-front propagation — unlike the mean-field ODE — and can exhibit extinction below the deterministic threshold. Click the grid to start an outbreak.