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\bibitem{Somaini_etal1999} Somaini, B., Wang, J., Perozo, M., Kuhn, F., Meili, D., Grob, P., et al. (1999). \textit{A continuing concern : HIV and hepatitis testing and prevalence among drug users in substitution programs in Zurich, Switzerland}. Zurich: Arbeitsgemeinschaft f{\"u}r risikoarmen Umgang mit Drogen (ARUD).

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