It is widely believed that HIV is predominantly sexually transmitted in Sub-Saharan Africa. This claim is inconsistent with national representative data from Lesotho, Zimbabwe, and Swaziland which reveals that a significant proportion of HIV infections occurred in adolescents who claim to be virgins. Two explanations for this observation have been proposed: adolescents misreport sexual status or non-sexual risks are more prevalent than previously asserted. This paper empirically uncovers the implicit assumptions underlying this discussion, by estimating the proportion of sexually transmitted HIV infections assuming that misreporting is irrelevant, and the proportion of misreporting necessary to conclude that HIV is predominantly sexually transmitted. It shows that under the no-misreporting assumption, 70% of HIV cases in the respective sample of unmarried adolescent women is not due to sexual transmission. The assumption that HIV is predominantly sexually transmitted is valid only if more than 55% of unmarried adolescent women who are sexually active have misreported sexual activity status. This research is designed to gain better understanding on the importance of different transmission modes. This is important to design combination prevention to achieve maximum impact on HIV prevention.