2.1 General approach. The approach we use here is very similar to the classical approach based on multifactorial logistic regression, which consists of: -Step 1: Choice of a model H 0 against which the data is compared. In the case of the classical logistic regression approach, it is assumed that all individuals have a same probability p to be infected, independently of the other individuals' status. As a result, under model H 0 the distribution of the number of infected cases in the population follows a binomial distribution of parameter p and N, where N is the number of individuals of the population. -Step 2: Some of the model parameters are expected to depend on risk factors.