The generation time is defined as the time between the infection of a farm and the time at which the farm passes on infection to another farm. We have assumed the generation time distribution is Weibull. While this is a biologically plausible choice, we cannot verify it empirically. As such, we assessed robustness to this choice using other plausible choices such as the gamma distribution (results not shown). However, the following results under these alternatives did not differ substantively from those shown below. Figure 2 shows the estimated generation time distribution for the four different outbreaks; the parameter estimates are detailed in Table 1 . The estimates, and hence the distribution, differs substantially between the outbreaks. It could be hypothesised that the generation time would shorten after measures were put in place to isolate the infected farms. However, allowing for different generation time distributions for the pre-and post-control time periods did not significantly improve the model fit. Table 2 ) with upper 95% bounds in the range 1.5-3.6.