Particles spread infection by physical proximity. Tune the transmission rate β, recovery rate γ, vaccination coverage and watch the reproduction number R₀ and epidemic curve evolve in real time.
SIR compartmental model: Susceptible → Infected → Recovered. The basic reproduction number R₀ = β/γ determines whether an epidemic spreads (R₀ > 1) or dies out (R₀ < 1).
Adjust β (transmission rate) and γ (recovery rate). Vaccinate a portion of the population to see herd immunity emerge. Watch the epidemic curve flatten.
Herd immunity threshold = 1 - 1/R₀. For measles (R₀ ≈ 15), 93% vaccination is needed. For seasonal flu (R₀ ≈ 1.3), about 23% is sufficient.