[영문]Most analyses of the determinants of HIV infection are performed at the individual level. The recent Demographic and Health Surveys, which include results from HIV tests, allow the study of HIV infection at the level of the cohabiting couple. This article exploits this feature of the data for Burkina Faso, Cameroon, Ghana, Kenya, and Tanzania. The analysis yields two findings about the dynamics of the HIVAIDS epidemic that have important implications for policy. First, at least two-thirds of the infected couples are sero-discordant, that is, only one of the two partners is infected. This implies scope for prevention efforts among infected couples. Second, among 30-40 percent of the infected couples only the woman is infected. This is at odds with levels of self-reported extramarital sex by women and with the common perception that unfaithful men are the main link between high-risk groups and the general population. These findings are confirmed by tests of robustness. These results indicate that extramarital sexual activity among women in union is a substantial source of vulnerability to HIV that should be, as much as male extramarital activity, targeted by prevention efforts.