First, we look at the distribution of FIV prevalence in males for arbitrarily fixed values of the parameters: b M = 0.025, M = 50 and e M = 0.01 (Fig. 3a, solid line) . For clarity, we plot the distribution of FIV prevalence as a continuous line, although the distribution is discrete. The probability of finding no infected cats in the population is high (17%). The mean prevalence of FIV is 12.66% and in 95% of the model outcomes the FIV prevalence ranges between 0 and 32%. This predicted distribution of FIV prevalence in males differs from a binomial one (a distribution frequently used in risk factor analysis, [8, 9] ) having the same mean (Fig. 3a , dashed line). For a binomial distribution, the probability of finding no infected individuals in the population is much smaller (0.1%) and the confidence interval for FIV prevalence is [0.01; 0.20].