Perspectives on the basic reproductive ratio

The basic reproductive ratio, R-0, is defined as the expected number of secondary infections arising from a single individual during his or her entire infections period, in a population of susceptibles. This concept is fundamental to the study of epidemiology and within-host pathogen dynamics. Most importantly, R-0 often serves as a threshold parameter that predicts whether an infection will spread. Related parameters which share this threshold behaviour, however, may or may not give the true value of R-0. In this paper we give a brief overview of common methods of formulating R-0 and surrogate threshold parameters from deterministic, non-structured models. We also review common means of estimating R-0 from epidemiological data. Finally, we survey the recent use of R-0 in assessing emerging diseases, such as severe acute respiratory syndrome and avian influenza, a number of recent livestock diseases, and vector-borne diseases malaria. dengue and West Nile virus.