Wave fronts on a 2D lattice — susceptible/infected/recovered dynamics
The SIR model partitions a population into Susceptible, Infected, and Recovered. R₀=β/γ is the basic reproduction number: R₀>1 causes an epidemic, R₀<1 causes die-out. On a 2D spatial lattice, infections spread as travelling wave fronts — a key insight for understanding geographic disease spread and herd immunity thresholds (1-1/R₀).