= Cooperative SIS coinfection threshold
{title2=$q>2,\quad2\sqrt{q-1}/q<\mathcal R_0<1$}
For two otherwise symmetric <SIS models> with recovery rate $\gamma$, primary transmission $\beta$ and secondary susceptibility multiplier $q$, split hosts into uninfected, singly infected and doubly infected classes. The displayed conditions give stable disease-free and joint endemic states, with an intermediate saddle: a strong <Allee effect>. The endemic susceptible fraction solves $(q-1)S^2-qS+\mathcal R_0^{-2}=0$. The smaller root is the stable high-prevalence state. Coinfected hosts recover each infection independently, so recovery from coinfection leads to a single-infection class rather than directly to complete susceptibility.
Back to article page
