Mathematical models will be developed for disease control through contact investigation, using individual- based simulations. Realistic contact investigation strategies will be analyzed for pathogens that may be transmissible from asymptomatic carriers as well as transmitted prior to symptoms. It will be determined how extensive or how targeted contact investigation should be, and what information should be systematically collected in future contact investigations. The models also include specific questions regarding the effect of behavior change during a contact investigation. Such models will be applied to tuberculosis transmission (because of the availability of contact investigation expertise and the availability of molecular epidemiogical data at the population level). Such models will also be applied in the setting of community-based trials of trachoma elimination, because such trials provide possibly the only experimental human settings wherein we may monitor the reemergence of an infectious disease. Specific testable predictions are outlined in the proposal. RELEVANCE: Our project is designed to improve planning for decision making and emergency preparedness by developing specific operational models to explore the best way to control epidemics using contact investigation (or related methods, including ring vaccination), and to improve data collection during contact investigations. We are proposing to use tuberculosis and trachoma in developing empirical case studies.