  The role of cooperative effects (i.e. synergy) in transmission of infection
is investigated analytically and numerically for epidemics following the rules
of Susceptible-Infected-Susceptible (SIS) model defined on random regular
graphs. Non-linear dynamics are shown to lead to bifurcation diagrams for such
spreading phenomena exhibiting three distinct regimes: non-active, active and
bi-stable. The dependence of bifurcation loci on node degree is studied and
interesting effects are found that contrast with the behaviour expected for
non-synergistic epidemics.
