where P[l] is a suitable probability distribution with mean l. The difference between this and that presented in [9] [10] [11] is that the number of locally acquired cases is used as the data at time t+t rather than all cases. The standard SIR model only deals with the average number of cases so a suitable probability distribution for P[l] is a Poisson distribution which is the most general form (highest entropy) if only averages are known.