In the contour plots in Figure 3 , tracing is ineffective in the dark grey areas (small latent period or large delay), so smallpox and SARS control are predicted to be able to cope with some delays, whereas it might be more of a problem with FMD and influenza. If forwards tracing is maximally effective and not affected by the delay, p c * = 121/R 0 pre = 0.33 as indicated by the light grey area (smallpox if d,0.5). If the detection time is variable, tracing may be effective even if a proportion less than 121/R 0 pre is traced (SARS if d,0.4, panel 3C). Because backwards tracing is needed to attain this result, it is only possible when the tracing delay is shorter than the infectious period (not shown in the Figures).