= Frequency-dependent transmission
{title2=$\beta SI/N$}
For <frequency-dependent transmission> with <homogeneous mixing>, each member of the <susceptible compartment> experiences <force of infection> $\lambda=\beta I/N=\beta i$. Consequently the total flow into the <infectious compartment> is $\beta SI/N$ people per unit time. The constant $\beta$ combines the contact frequency and the chance that a relevant contact causes <infection>, and has units of inverse time. Holding the <infectious compartment>'s <fraction> fixed holds each person's <force of infection> fixed when $N$ changes. An unnormalized <mass-action infection> convention instead writes the total flow as $bSI$, where $b=\beta/N$ has units of inverse person-time; these two coefficients must not be confused.
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