Modelling transmission: mass action and beyond

In a recent review in TREE, McCallum et al. say that they are confused by the terminology related to mass action in disease transmission that we introduced in Ref. In retrospect, we see that our description of the expression ‘βSI’(where S is the number of susceptible hosts, I is the number of infected hosts and β is the transmission coefficient) using the words ‘pseudo mass-action’ (thus implying that the description is not a correct rendering of mass action) might be confusing when one does not restrict this, as we did, to a case where S and I are numbers