🧪 SEIR Epidemic & Herd Immunity Threshold Simulator

This simulator integrates the real SEIR compartmental model — Susceptible, Exposed, Infectious, Recovered — as coupled differential equations, letting you set the transmission rate, incubation period and infectious period and see the resulting outbreak curve emerge from the actual math rather than a scripted animation. A pre-outbreak vaccination slider moves that share of the population directly into the immune compartment before the first case, so you can watch the infectious curve shrink and, once coverage crosses the model's own computed herd-immunity threshold p_crit = 1 − 1/R₀, collapse into a non-event — a genuine emergent result of the integration, not a canned effect.