There are known weaknesses in SIS-descended models, some of which are discussed by Hyman and LaForce [14] . They suggested that a model dealing with demographics and their subgroups would be useful and described a start Theoretical Biology and Medical Modelling 2006, 3:32 http://www.tbiomed.com/content/3/1/32 toward conceiving such a model, creating a matrix of SIRP flows for each demographic group within a "city" and modeling contacts between these groups. Thus, the possibility of building an entirely discrete model using the object-oriented approach, essentially setting the granularity of the Hyman-LaForce concept at the level of the individual, together with Monte Carlo method, was attractive. The object method of design seemed to be a good fit, since object-oriented programming was invented for discrete simulations [24] . An Object-Oriented (OO) design defines as its primitive elements "black box" subunits that have defined ways of interacting with each other [25] .