Each of these individual components, when combined, represents a comprehensive HIV prevention program. Surveillance efforts track and monitor the epidemic, guiding research questions and policy development and allowing prevention efforts to be targeted to areas where they are needed the most. Ongoing research contributes to a better understanding of the complex factors that lead to HIV infection and helps to identify effective approaches to preventing infection. Intervention activities are designed to change risky behaviors and improve the health of the people served, and therefore work to contain the spread of the disease. Capacity building efforts ensure that organizations implementing HIV prevention programs are equipped with the necessary training, information, and skills to implement effective programs. Evaluation of prevention programs allows CDC to monitor progress and refine its efforts by identifying the most effective programs. Finally, policy research and development identifies critical policy questions and underpins national policies that support and facilitate HIV prevention efforts.