Model infection dynamics with basic parameters.
The SIR model divides the population into Susceptible, Infected, and Recovered. Herd immunity threshold = 1 − 1/R₀. For COVID-19 (R₀ ~2.5): HIT ~60%. For measles (R₀ ~15): HIT ~93%. The model assumes homogeneous mixing and permanent immunity. Real-world dynamics involve heterogeneity, waning immunity, and behavioral changes.