Basic reproduction number for in-host viral infections

In epidemiology, the basic reproduction number ($R_0$) is used to measure the spread potential of a disease. It is defined as the number of secondary infections produced by a first infection in a homogeneous susceptible population. To calculate $R_0$ for within host models, modelers use the methods commonly used for epidemiological models. However, the infection dynamics of epidemiology models differ from the simple in-host viral model in that epidemiological models assume direct contact between infected and susceptible individuals while in-host infectious use the virus as an intermediary agent. We examine the meaning of the basic reproduction number in the context of a simple in-host viral infection model.