Figure 2. Summary of the assumptions defining contact between persons in the model. A, B, and C show the annual risk for infection, estimates of the average effective contact number in the model, and the average age-specific annual incidence of infectious disease per 100,000 population respectively in the various settings. For settings in which the annual risk for infection has not changed over time, the effective contact number is obtained from the ratio between the annual risk for infection and the incidence of infectious cases predicted in the model. The values for the effective contact number in the Netherlands are identical to those calculated in reference 6. D shows the frequency distribution of the assumed number of persons effectively contacted by each infectious case-patient, if the population were to comprise 1,000 infectious cases and the average effective contact number was approximately 4, as assumed for the Netherlands for recent years. This (negative binomial) distribution (defined by a variance 20 times the mean) led to observed cluster distributions that best compared against those observed in the Netherlands (6). Contact between persons is assumed to be assortative (so that, for example, those with a high-risk lifestyle, mix preferentially with similar persons) and, for simplicity, independent of age and sex.