In this section, we identify some of the more popular methods (although by no means all) used to calculate R 0 . We also describe the limitations that each method presents and demonstrate one of the core problems with R 0 . Specifically, we address a key problem with R 0 : how do biologists make sense of it from mathematical models? (See, for example, the puzzled discussion in van den Bosch et al. [6] .)