Although the "R" in R 0 is derived from "reproductive", based on the original formulation of the concept as the average number of secondary infections, many thresholds have been denoted by "R 0 ", even when they are not related to the average number of secondary infections. Thus, in keeping with the notation, we will use the notation R 0,X to denote an R 0 -like surrogate associated with a particular method, symbolised by X. The survival function has the advantage that it always produces the average number of secondary individuals infected by a single infected individual, in the same class. Thus, in Figure 1 , where one human infects two mosquitoes, who each infect three humans, the survival function produces R 0 = 6. This is the number of humans infected by a single infected human via mosquitoes; or, equivalently, the number of mosquitoes infected by a single mosquito via humans. The survival function is a generalised method of calculating the basic reproductive ratio that is not restricted to ODEs.