Again, in the special case S=N?1, the branching-process approach can be extended to derive the probabilities of outbreak sizes before emergence (see Text S1, A.1.2, for more details). In general, the probability of an outbreak of size x i is defined as g i (I i,0 , x i ) with i denoting the strain. The number of infected hosts to start with is denoted by I i,0 . Furthermore, the overall outbreak size probability can be derived using conditional probabilities p out (X )~g 1 (I 1,0 , x 1 ) X I 1,0 zx 1 ,...,I n{1,0 zx n{1