This has the same threshold property as R 0,N = βS/μ, but is clearly not the average number of secondary infections. In essence, the next-generation method is a mathematical generalisation of putting the negative values on one side and dividing (this is what V −1 is) so that the eigenvalue threshold at zero is transformed into an R 0 -like threshold at one. However, as we have seen, this does not produce a unique result. van den Driessche and Watmough [10] note that other decompositions of F and V can be chosen, which lead to different values for R 0,N . They claim that only one choice of F is epidemiologically correct. However, this is not true, as the above example shows. Furthermore, it means that the definition of R 0 relies upon the judgement of the modeller as to what "epidemiologically correct" means.