The social behavior and the evolution of sexually transmitted diseases

We introduce a model for the evolution of sexually transmitted diseases, in which the social behavior is incorporated as a determinant factor for the further propagation of the infection. The system may be regarded as a society of agents where in principle, anyone can sexually interact with any other one in the population, indeed, in this contribution only the homosexual case is analyzed. Different social behaviors are reflected in a distribution of sexual attitudes ranging from the more conservative to the more promiscuous. This is measured by what we call the promiscuity parameter. In terms of this parameter, we find a critical behavior for the evolution of the disease. There is a threshold below which the epidemic does not occur. We relate this critical value of promiscuity to what epidemiologists call the basic reproductive number, connecting it with the other parameters of the model, namely the infectivity and the infective period in a quantitative way. We consider the possibility of subjects to be grouped in couples.