An epidemic burns out before it reaches everybody, and that is not luck. The SIR model of Kermack and McKendrick, published in 1927, splits a population into three compartments: susceptible people become infectious at rate β and then recover at rate γ, so the ratio R₀ = β/γ decides everything. Below 1 the chains of transmission break by themselves; above 1 the wave climbs to a peak and falls again as susceptibles run out. It is a deliberately simplified model assuming uniform mixing and lasting immunity, not a forecast of any real outbreak.
An epidemic burns out before it reaches everybody, and that is not luck. The SIR model of Kermack and McKendrick, published in 1927, splits a population into three compartments: susceptible people become infectious at rate β and then recover at rate γ, so the ratio R₀ = β/γ decides everything. Below 1 the chains of transmission break by themselves; above 1 the wave climbs to a peak and falls again as susceptibles run out. It is a deliberately simplified model assuming uniform mixing and lasting immunity, not a forecast of any real outbreak.
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