We read with great interest ‘Polygyny and the spread of HIV in sub-Saharan Africa: a case of benign concurrency’ [1]. The authors’ demonstration that communities in which formal polygyny is common tend to have lower HIV infection rates is convincing, but also mysterious. Although the authors state that the relationship between HIV and polygyny at the individual level is ‘inconclusive’, many previous studies including those of the authors themselves have found that, within a given geographical area, men and women in polygamous marriages tend to have more extramarital sex and HIV and sexually transmitted infection (STI) [2–9]. We are unaware of any studies that show polygyny is protective at the individual level, while controlling for such potential confounders as male circumcision, urban residence, nationality, tribe or religion. How can polygyny be a risk factor at the individual level, but not at the community level? One hypothesis is that cultural attitudes to women serves as a confounder of the relationship between the prevalence of polygyny and the prevailing sexual networks through which HIV spreads. Communities where formal polygyny is common, such as those in some West African countries, might be more conservative, and more similar to the Asian or Middle Eastern pattern in which women's roles tend to fall into one of two categories: faithful dependent wives or largely urban prostitutes. In such cases, the spread of HIV is largely confined to sex workers and their clients, and the epidemic in the general population is limited. However, compared with people in monogamous unions, polygamous people might still be individually more vulnerable to infection, especially if they are more likely to be unfaithful, because they are put at risk not only by their own infidelity, but also by that of all the others in the marriage. In regions such as Southern Africa where traditions of formal polygyny have been eroded over generations, in part by urbanization and other economic upheavals, women's identities are more fluid, and many play the role of ‘semi-wives’ forming casual but frequently transactional, longer-term liaisons with multiple men who provide them with material support of various kinds [10]. One indication of this is the growth of the ‘ambiguous conjugal union’ described by Bledsoe [11], which is increasingly the norm in urban Africa. We note that the authors found that urban residence was an independent risk factor for HIV infection. Women in such unions have many fewer partners than ‘professional’ prostitutes do and the meaning of ‘transactional sex’ is also very different. An exchange of money or gifts tends to signify commitment, not a one-off exchange with someone they never intend to see again. As these women recognize that men are more likely to support a child of their own, rather than a temporary girlfriend, some urban African women practice ‘polyandrous’ motherhood, to secure support from several men. This pattern of sexual relations differs radically from the more conservative polygamous cultures of the Middle East or rural West Africa, and the more open sexual networks have very different implications for the spread of HIV. The overlapping, if temporary, longer-term relationships have the potential to create an interlocking network of sexual relationships, in which the ‘reachable paths’ for the virus are extensive and durable. Reniers and Watkins consider but reject the possibility that greater surveillance of women within the polygynous cultures of West Africa could explain their findings because they control for prevalence of extramarital sex among women. But this would not exclude our interpretation for two reasons. First, in the low-polygyny Southern African countries there is not much marriage at all, hence relatively little ‘extramarital’ sex. Second, the Demographic and Health Surveys the authors use are subject to considerable under-reporting of multiple partnerships, especially for women [12]. We appreciate the author's treatment of polygyny as a subset of concurrent partnerships and agree that in theory a closed polygynous relationship could be as insulated from STIs and HIV as a mutually monogamous one. Nevertheless, it is worth noting that even within ‘low HIV/high polygyny’ communities, infidelity is still strongly associated with infection. Thus, infidelity – a subset of concurrent partnerships – remains as risky for polygynous partnerships as it is for monogamous partnerships, if not more so. Acknowledgements The views and opinions expressed in this letter are solely those of the authors and do not necessarily reflect those of the US Agency for International Development nor those of the US Government.