2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/85158In this work, we revisit the basic reproduction rate $\mathcal{R}_{0}$ definition for analysis of epidemic-non-epidemic phases describing the dynamics of the discrete stochastic version of the epidemic $SIR$ model based on the Master Equation formalism. One shows that it is a very precise and efficient way to determine the epidemic threshold; using its most primitive concept, we can find exact results.Submitted to PRLBiological PhysicsQuantitative Biologytext