Maintenance of endemicity in urban environments: a hypothesis linking risk, network structure and geography.

In industrialised countries, a rapid epidemic phase of HIV transmission has largely given way to more moderated endemic transmission. The dynamics of endemic transmission may differ substantially from those generating epidemic spread. We hypothesise that three elements play an important role in maintaining endemicity in high prevalence urban environments. First, persons are likely to be subject to multiple risks from multiple sources rather than engaging in a single, hierarchically classified, risk behaviour. Second, the network structure in these environments may include a substrate of "fixed" factors (a large connected component, a characteristic degree distribution and small world phenomenon) upon which is superimposed a number of variable factors (transitivity, assortativity) that determine the level of prevalence. Third, the geographic range of persons in these milieux is constricted, making it likely that new partners will already be connected. The confluence of these three factors assures the ongoing risk bombardment needed for maintenance of endemicity. Further empirical and theoretical analysis will be required in order to validate this hypothesis.