The many guises of R0 (a didactic note).

The basic reproduction number R0 is, by definition, the expected life time number of offspring of a newborn individual. An operationalization entails a specification of what events are considered as "reproduction" and what events are considered as "transitions from one individual-state to another". Thus, an element of choice can creep into the concretization of the definition. The aim of this note is to clearly expose this possibility by way of examples from both population dynamics and infectious disease epidemiology.