The paper provides an overview of the basic reproductive ratio ($R_0$), a fundamental concept in epidemiology and within-host pathogen dynamics. $R_0$ is defined as the expected number of secondary infections arising from a single individual during their entire infectious period in a susceptible population. The authors discuss various methods for formulating $R_0$ from deterministic, non-structured models, including the survival function method and the next generation operator. They also review common techniques for estimating $R_0$ from epidemiological data, such as using the endemic equilibrium, the final size equation, and the intrinsic growth rate. The paper further examines the recent use of $R_0$ in assessing emerging and endemic diseases, including SARS, avian influenza, bovine spongiform encephalopathy (BSE), foot and mouth disease (FMD), and vector-borne diseases like malaria, dengue, and West Nile virus. The authors highlight the importance of $R_0$ in guiding public health measures and control strategies.The paper provides an overview of the basic reproductive ratio ($R_0$), a fundamental concept in epidemiology and within-host pathogen dynamics. $R_0$ is defined as the expected number of secondary infections arising from a single individual during their entire infectious period in a susceptible population. The authors discuss various methods for formulating $R_0$ from deterministic, non-structured models, including the survival function method and the next generation operator. They also review common techniques for estimating $R_0$ from epidemiological data, such as using the endemic equilibrium, the final size equation, and the intrinsic growth rate. The paper further examines the recent use of $R_0$ in assessing emerging and endemic diseases, including SARS, avian influenza, bovine spongiform encephalopathy (BSE), foot and mouth disease (FMD), and vector-borne diseases like malaria, dengue, and West Nile virus. The authors highlight the importance of $R_0$ in guiding public health measures and control strategies.