Opinion Introducing the Outbreak Threshold in Epidemiology

Abstract: When a pathogen is rare in a host population, there is a chance that it will die out because of stochastic effects instead of causing a major epidemic. Yet no criteria exist to determine when the pathogen increases to a risky level, from which it has a large chance of dying out, to when a major out-break is almost certain. We intro-duce such an outbreak threshold (T0), and find that for large and homogeneous host populations, in which the pathogen has a repro-ductive ratio R0, on the order of 1