This week, we introduce the basic Susceptible-Infected-Recovered model framework to describe an epidemic in a susceptible population. We walk through the basic equations, and the parameters that describe the dynamics of the pathogen in the host and the resulting epidemiology of the epidemic in the population. We introduce the concept of the basic reproduction number, R0, and discuss how it relates to the parameters and structure of the SIR model. We begin to discuss how interventions are modeled in the context of SIR models, including vaccination.