The continuing HIV/ AIDS pandemic represent one of the greatest challenges for researchers and clinicians. This book gives an overview of HIV epidemiology and virology while discussing high-risk behavior factors, HIV immunity and diagnosis. The author argues that vaccine studies usually require volunteers who are followed over a long period of time. She further shows that a high risk cohort of exposed seronegative individuals, that was studied in Nairobi - Kenya, could provide background data for planning HIV-1 Phase II and III vaccine trials. The book discusses challenges encountered in the follow-up of such a cohort in a developing country where such studies have not been done before. The author also explains that the measurement of cellular immunogenicity by interferon gamma Elispot assay in exposed seronegative individuals was not effective. Consequently, there is need to improve the assay or use it in combination with other assays. This book is recommended to students and researchers in biomedical sciences especially those interested in HIV vaccinology.