Several strategies are possible to deal with the fact that one never observes infections, but rather as a time series of cases of the form C i , where case definitions could be based on symptoms, hospitalisation or seroconversion. One strategy, used in [12] , is simply to ignore this and use cases as surrogates of infection for estimation of both the generation time and the reproduction numbers. Often though, it may be possible to characterise a distribution of the time from infection to becoming a case, say j i where P n i~0 j i~1 . If a case is defined by symptoms then this would be the incubation period distribution. One can then backcalculate incidence as follows