Modeling prevention interventions requires insights into epidemiological and biological factors affecting HIV transmission. Project 1 of the proposed Program Project addresses the epidemiological determinants of HIV transmission by examining the per contact transmission probability (P), the rate of partner change/ contact rate (c), and the duration of infectiousness (D), which all contribute to the reproductive number (R0= pcD), defined as the average number of secondary infections arising from a single infection in a completely susceptible population. To achieve this objective, we will address the following specific aims: Aim 1: Per contact transmission probability (P) - The per contact sexual transmission probability of HIV will be estimated, taking into account stage of HIV infection in the source partner, presence of viral and bacterial sexually transmitted infections, circumcision status, and other biological cofactors investigated in Project 2. The absolute and relative difference of transmission probability for insertive versus receptive anal sex will also be considered. Aim 2: Rate of partner change (c) - The role of partner selection in HIV acquisition and transmission risk will be examined at individual, partnership, and network levels. At the individual level, the relative contribution of HIV risk due to number of sexual acts versus number of sexual partners will be examined. At the partnership level, risk characteristics will be compared between transmitting and seroconcordant non-transmitting partnerships. At the network level, risk of acquisition and transmission of HIV based on the location of individuals within networks of venues used to meet sex partners, and correlates of being in a transmission cluster, will be elucidated. Aim 3: Duration of infectiousness (D) - To determine how duration of the infectious stages of HIV infection intersects with risk behaviors and time to antiretroviral treatment. We will use data from Aims 1-3 to develop a stochastic, individual-based model to predict the number of secondary infections arising from individuals diagnosed during recent HIV infection as they transit to chronic infection, and the potential impact of short-course antiviral therapy on reducing transmission. Project 1 depends on well-characterized clinical data and specimens collected by the Clinical and Specimen Core and the subsequent measurement of transmission correlates in Project 2. Epidemiological data contributing to HIV transmission collected in Project 1 will help to define correlates of HIV transmission in Project 2. The Administrative Core will coordinate administrative, fiscal, data, and statistical support for Projects 1 and 2. These data will provide critical information on epidemiological determinants of HIV transmission in a setting where HIV-1 subtype B predominates, which are lacking for MSM, and will generate important exploratory data for other populations.