We see that the well known multiplicative progression between new cases at successive times due to contagion appears, on average, as a linear relation between DT(t+t) and DT(t) in an epidemic time delay diagram, Figures 2a-d. Expression [6] generalizes similar relations in the TSIR literature by casting them in terms of new cases over arbitrarily chosen observation intervals t, not necessarily coinciding with the average generation time c 21 . Expression [6] also shows how the initial R t can be estimated geometrically (without the need for parameter search or numerical optimization) from an epidemic time delay plot of surveillance data: b(R t ) is the slope of the tangent at the origin of case trajectories (dashed line in Fig. 2a, b) . For emerging infectious diseases relative fluctuations in case numbers are large, see e.g. Figure 2d , and this simple geometric approach is not valid, thus making more robust estimation methods, as the one presented here, necessary.