Injecting in a park was another risk factor for HIV infection. It is well established that the size and density of social and injecting networks are key factors in the transmission of HIV among IDUs [12,13]. Indeed, a recent study in Pakistan pointed out large sharing networks as a key factor for explosive HIV prevalence growth among IDUs [14]. Assuming that IDUs who recently injected in parks did so in the past as well, the high prevalence of HIV found among those injecting in parks plausibly reflects their exposure to larger and more diverse needle sharing networks than those not injecting in parks.