On collective epidemic models and (partially) exchangeable events

Recently, Picard and Lefèvre introduced a new epidemic model, called the collective epidemic process, to describe the spread of S-I-R infectious diseases. The model there was built mainly from analytical considerations. The primary motivation of the present work is to point out that its construction can be derived from a simple and general probabilistic argument. This leads us to have a further look at a classical elementary subject in probability, namely the realization of events among n (partially) exchangeable events.