is the size-biased household size distribution. Given a household of size n gets infected, the probability of a chain of infections is given by the Reed-Frost probabilities pr ({m 1 , …, m n }|n). The distribution of the random variables X = {n, m 1 , …, m n } at infection is thus f X ð Þ~k n : pr m 1 , . . . ,m n f g j n ð Þ ð 30Þ