Suppose there are N infected farms, labelled i = 1,…,N, and ordered so that the first k farms are those that contracted their infection from outside sources. The infection times of these farms are t = (t 1 ,…,t N ) such that t 1 = … = t k = 0. Under the simplest model that neglects any differences between farms, spatial locations, etc., the probability that farm j[fkz1,:::,Ng was infected by farm i[f1,:::Ng is