Incidence periodicity of the model is much less sensitive to cross-immunity and age structure, resulting in a wide region of close fitting behaviour for values of u between about 0.2 and 0.6 and f between 0.3 and 0.5. As discussed in §3, this strongly suggests that an agestructured population and, in particular, a pathogen population with more than one strain and crossimmunity, are necessary to reproduce the patterns of behaviour found in the ILI time series. Models based on single strains and well-mixed populations generate annual and biennial behaviour for the same parameter values. The necessity of multiple strains has been noted in other work modelling seasonal influenza [14] , although in that work the two strains did not interact.