Susceptible compartment
= Susceptible compartment
{title2=$s=S/N$}
The <susceptible compartment> consists of people who can acquire <infection>. In the closed <SIR model>, $S(t)$ is their count and $s(t)=S(t)/N$ their <fraction> of the fixed total $N$. Membership expresses the assumptions of the <mathematical model>, rather than measuring the strength of an individual's <immunity>.