Gillespie algorithm — exact stochastic simulation of epidemic dynamics
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Susceptible
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Infected
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Recovered
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R₀
The Gillespie algorithm simulates exact stochastic trajectories. Each reaction (infection or recovery) occurs at a random exponential time. R₀=β/γ: outbreaks occur when R₀>1, but with small I₀ stochastic extinction is likely even when R₀>1. Multiple runs (Add Run) show trajectory variability.