Survival and extinction of epidemics on random graphs with general degrees

Abstract: In this paper, we establish the necessary and sufficient criterion for the contact process on Galton-Watson trees (resp. random graphs) to exhibit the phase of extinction (resp. short survival). We prove that the survival threshold $\lambda_1$ for a Galton-Watson tree is strictly positive if and only if its offspring distribution $\xi$ has an exponential tail, i.e., $\mathbb{E} e^{c\xi}<\infty$ for some $c>0$, settling a conjecture by Huang and Durrett [12]. On the random graph with degree distribution $\mu$, we show that if $\mu$ has an exponential tail, then for small enough $\lambda$ the contact process with the all-infected initial condition survives for $n^{1+o(1)}$-time w.h.p. (short survival), while for large enough $\lambda$ it runs over $e^{\Theta(n)}$-time w.h.p. (long survival). When $\mu$ is subexponential, we prove that the contact process w.h.p. displays long survival for any fixed $\lambda>0$.