Population structure and change and social contact patterns are major determinants of the observed epidemiology of infectious diseases, including the consequences on health. Demographic structure and the components of demographic dynamics are changing over time and substantially differ within countries and most critically between countries. However, some of the overall consequences of demographic changes remain unclear, though urbanisation and fertility decline will certainly have a profound impact on social structures, family composition and, as a consequence, on disease spread and on the identification of effective public health measures.DECIDE will explore the following questions:1. What are the major short- and medium-term impacts of demographic changes on the patterns of infectious disease (morbidity and mortality)?2. How are these demographic changes affecting contact patterns that are of fundamental importance to the spread of infectious diseases? Are there new and different modes of transmission within and between populations?3. What are the implications of demographic changes for infection control strategies? What is the interplay between demographic changes and public health policies in shaping future trajectories of infectious diseases?In order to answer these questions, DECIDE will use the following strategy: analyse harmonised demographic and health survey data (DHS), and health and demographic surveillance system data (HDSS); develop new estimates of social contact patterns and other socio-demographic variables collecting data from representative samples of both urban and rural settings in selected countries; develop a theoretical framework to predict the likely chains through which demographic change influences the burden of infectious diseases; develop and parameterise mathematical population models for the transmission of infectious diseases to evaluate the impact of public health measures under changing demographic conditions.