UNAIDS defines “risk” as “the probability that a person may acquire HIV infection,” usually as a result of specific behaviour s that allow HIV transmission to occur (UNAIDS, 2007).  By contrast, an individual is “vulnerable” to HIV when his or her ability to avoid infection is diminished by one or more other factors, such as lack of personal knowledge or skills, the influence of cultural norms that validate risky behaviours, or physical surroundings that make risk reduction difficult or impossible (UNAIDS, 2007; see Bates et al., 2004).

Fully comprehending the HIV-related risks and vulnerabilities of many key populations is challenging.  For many key populations – such as sex workers, men who have sex with men, and people who inject drugs – there is no clear understanding of the size and distribution of these groups.  Moreover, as these groups are socially marginalized and subject to criminal penalties for engaging in the behaviours that define them for epidemiological purposes, it has often been difficult to conduct relevant studies to quantify HIV prevalence or identify key factors associated with increased risk of infection.