Epidemic threshold for the closed SIR model
= Epidemic threshold for the closed SIR model
{title2=$\mathcal R_0=\beta S_0/\gamma$}
The deterministic <SIR model> with initial susceptible count $S_0$ grows initially exactly when $\beta S_0>\gamma$. Its <basic reproduction number> is $\beta S_0/\gamma$ under unnormalized <mass-action infection>. If that ratio is at most one, decreasing $S$ prevents later infectious growth.