Discovering an epidemic before it has reached a certain level of prevalence

In this communication we calculate the probability of discovering a simple epidemic in a large population before the epidemic has reached a given level of prevalence, by regularly taking a small random sample from the population for microbiological screening. Apart from the general formula which has to be evaluated numerically, we derive various simple approximation formulae which shed light on the properties of various monitoring regimes. These formulae are, moreover, rather robust against deviations from the model specifications. The results are applied to the evaluation of the efficiency of an infection-monitoring program in an animal breeding centre.