The effect of a delay in each tracing step was studied by determining the critical tracing probability p c * for R 0 pre = 1.5, as a function of the latent period and tracing delay. Figure 3 shows the resulting contour plots for the four special cases with singlestep tracing, and it indicates the approximate positions of the four real infections. Along the t lat axis (d = 0) lie the R 0 pre = 1.5 curves of Figure 2 .