The probability of different chains of infection within households can easily be computed from the assumed Reed-Frost model [2] . I denote pr( {m 1 , m 2 , …, m n }|n) the probability of a chain of infection occurring in a household of size n where m 1 index cases infects m 2 , who in turn infect m 3 tertiary cases and so on, up to a maximum of n generations of infection. It is an assumption of the where pr m iz1 j m 1 , . . . ,m i f g ,n ð