The choice of which contact–density function to use in a model of disease transmission entails potentially significant consequences . Most importantly, the two functions differ in whether or not an infection is expected to persist below a critical density of individuals . The basic reproductive number (R0), defined as the number of secondary infections arising from the introduction of one infectious individual into a completely susceptible population, is a central epidemiological measure that characterizes the spread of an infection, and provides an immediate approximation as to how rapidly an infection can spread . In its simplest form, R0 = βN, where N is the population size and β is the transmission coefficient that consists of the rate p of becoming infected through contact with an infectious individual, multiplied by the contact–density function that equals cN/A (where A is area) when linear (density-dependent) and c when constant (frequency-dependent), and random homogeneous mixing is assumed . When transmission coefficient β changes with density, theory predicts a density below which the transmission rate is too low, causing R0 to fall below 1 and the disease to disappear, whereas no persistence threshold density exists when β remains constant, independent of density . Because changes in the transmission coefficient determine how quickly an infection can spread through a population, it can also be expected that the two contact–density functions will differently affect outbreak characteristics such as incidence, prevalence and outbreak size .