In Philadelphia, some drug users are selling clean needles from needle exchange programs on the street. Researchers say the black market isn't necessarily a bad thing. ImageZoo/Corbis hide caption

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A heroin user keeps a syringe tucked behind his ear at a park in the city of Medan on Indonesia's Sumatra island. Cordita-Caritas Medan, a nongovernmental organization active there, works to reduce HIV infections through rehab of drug users and a needle exchange program. Sutanta Aditya/AFP/Getty Images hide caption

itoggle caption Sutanta Aditya/AFP/Getty Images