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Let $i=I/N$ be the infectious proportion in a closed population. With homogeneous frequency-dependent transmission rate $\beta$ and recovery rate $\gamma >0$, the SIS model isThe basic reproduction number is the expected number of secondary infections produced by a typical newly infected individual in an otherwise susceptible population under the specified contact conditions. Here the infectious duration has mean $1/\gamma$, so $R_0=\beta /\gamma$.

For $R_0>1$ the attracting endemic equilibrium has $i_{\ast }=1-\gamma /\beta$. The homogeneous SIS susceptible fraction is thereforeFor $R_0\le 1$ the physical equilibrium is disease-free, $i_{\ast }=0$, $s_{\ast }=1$. At equality approach to zero is algebraic. Thus $s_{\ast }=1/R_0$ is an endemic relation, not a formula demanding more than one susceptible person per person below the invasion threshold.

The high-contact assumptions give $A>D$ away from a degeneracy. Since $C(p)=Bp+O(p^2)$, perturbing the larger eigenvalue in the assortatively mixed two-risk-group SIS model givesFor the biologically useful high-risk-core regime $A>\gamma >D$, put $x_0=1-\gamma /A$. Substituting $x=x_0+p{x}_1+O(p^2)$ and $y=p{y}_1+O(p^2)$ into the two equilibrium equations givesThus the group susceptible fractions and their population-weighted total areThe small-core SIS endemic expansion shows why the homogeneous SIS susceptible fraction relation fails for a mixed population: as the high-risk fraction tends to zero, $s_{\mathrm{total}}\to 1$ while $R_0\to A/\gamma >1$. A small high-risk core sustains infection even though almost everyone in the population remains susceptible. Even $s_H$ need not equal $1/R_0$ at first order.

For completeness, if $A,D<\gamma$ with a fixed gap from threshold, small $p$ gives disease-free equilibrium and total susceptibility one. If $D>\gamma$, the low-risk group already sustains infection. Put $y_0=1-\gamma /D$ and let $x_0\in (0,1)$ be the positive root of $(1-x_0)(A{x}_0+B{y}_0)=\gamma x_0$. The first-order coefficients areAgain it tends to $\gamma /D$ rather than $\gamma /A=1/R_0(0)$. These regular expansions require fixed nonzero gaps. At $A=D$ the spectral splitting is generally order $\sqrt{p}$; at $D=\gamma <A$, low-risk infection is also order $\sqrt{p}$, so a regular first-order Taylor expansion is inappropriate. An exact endemic replacement for the scalar identity is that the susceptible-weighted next-generation matrix has spectral radius one.