Hope-Simpson reported susceptible-infectious escape probabilities of Q = 69.9% for mumps, Q = 39% for varicella, and Q = 24.4% for measles in under 15s [1] . The results were reported independent of household size, and were regarded as unreliable in over-15s. Based on applying the Reed-Frost model to the measles (t) ) is shown, as is the infectiousness of the typical infected household (denoted b * (t * )). This latter curve is obtained by simulating over 10,000 epidemics of transmission within households starting from one infected case. The two analytical approximations described in the text are also shown. ''Approx 1'' is the main approximation described, while ''Approx 2'' is the one obtained by assuming that all infections occur in the second generation of infection within the household. Parameters are as described in the main text, and the curves are arbitrarily scaled such that each individual infects on average one person outside of the household (i.e. R G = 1). doi:10.1371/journal.pone.0000758.g002 estimate with this distribution of households, I obtain estimates of the average number of infections in each generation of infection of m 1 u 1, m 2 = 2.01 (i.e. the first index case directly infects an average of 2.01 people in his or her household), m 3 = 0.50, m 4 = 0.020, m 5 = 0.00036 and m 6 = 0.0000031, and thus the estimate for the total expected number of cases in an infected household is m = S 6 i = 1 m i = 3.54, to be compared to the mean size of 3.66. Hope-Simpson also reported the intervals between linked cases in households using different case definitions [1] ; the intervals for what he regarded as the most reliable case definition, ''maximum rash''. These data is well described by a Gamma distribution (not shown). The maximum likelihood estimate of the generation time is T g = 10.5 days with standard deviation 2.4 days.