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This article presents case histories of five cities in which HIV-1 has been introduced into a heterosexual injection drug user (IDU) community, but where HIV-1 seroprevalence has remained low and stable. Five cities were identified that met the operational definition for stable low HIV-1 seroprevalence, they are: Glasgow, Scotland; Lund, Sweden; Sydney, New South Wales, Australia; Tacoma, Washington; and Toronto, Ontario. In each of these five cities there were at least two studies of HIV-1 prevalence among IDUs not in treatment. The minimum sample size for determining seroprevalence for a given year in these studies was at least 95 subjects in each of the treatment and non-treatment samples. Local experts completed questionnaires describing the characteristics of the local drug-injection situation and the local AIDS prevention activities for IDUs. All of these local experts have been conducting research on HIV-1 infection among IDUs in their communities over the last five or more years.

This article, drawing on survey findings from the World Health Organization (WHO) Multi-City Study on Drug Injecting and the Risk of HIV Infection, describes the sexual behaviour of opioid and cocaine injectors in London, England, Toronto, Ontario, and Santos, Brazil.Not only are there behavioural differences in patterns of drug injecting between injecting drug users (IDUs) in these cities, but there are also social, cultural and economic differences. Most importantly, Santos is a city within a "developing" country where public health resources and infrastructures are constrained in ways which are uncommon to either London or Toronto. These differences at the city level allow for a comparative description of the sexual risk behaviour of drug injectors in different behavioural and cultural contexts. High levels of sexual activity and partner change in combination with high levels of unsafe sex clearly encourage a higher likelihood of HIV-1 transmission, particularly in cities where HIV-1 prevalence is high or shows no signs of decreasing.