The theory of island biogeography, seminal to modern ecological thinking, has been invoked hundreds of times to explain colonization sequences and species richness on "islands" of many sizes and in many locations. These applications have overwhelmingly focused on macro-organisms (among them lizards, birds, and beetles) and practically ignored microorganisms. In this project investigators at four U.S. research universities -- Old Dominion University, University of Georgia, University of Minnesota at Duluth, and University of Connecticut -- will apply the theory more provocatively: to infectious disease ecology. They argue that organic-rich marine and freshwater aggregates (synonyms include: marine snow, freshwater snow, organic detritus, and bioflocs) serve as "microscopic islands" for bacteria in aquatic environments. The theory's relevance in this context will be tested through development of analytical models parameterized by empirical studies in both field and laboratory. Using a three-step approach, the research team (microbial ecologists, infectious-disease modeler, shellfish biologist/ physiologist) together will: (1) Design and experimentally test an analytical model of island biogeography applied to individual aggregates ("islands") to predict bacterial species richness as a function of aggregate size and distance from a point source of pathogens (Species richness of marine aggregates); (2) Use the outputs from the species-richness model to construct and experimentally analyze a second mathematical model that considers the persistence of pathogenic species within the microbial communities of aggregates. The effects, if any, of species richness on persistence will also be determined (Pathogen persistence in marine aggregates); and (3) Apply information from the species-richness model and the pathogen-persistence model to evaluate links between marine aggregates and public health with an agent-based simulation model. The long-term goal of the principal investigators is to bring a community-level approach to studies of infectious diseases in aquatic environments. The broader impacts of this proposed research are many. Graduate students at all four participating universities will be trained, and two post-doctoral researchers will be supported and mentored. In addition, ambitious outreach efforts will develop innovative, inquiry-based teaching materials for students from grades K-12. These materials advocate basic ecological principles of infectious diseases, helping to fill a void in resources available for this topic. Between them, these institutions host more than 30,000 school children and educators annually, providing the NSF Ecology of Infectious Diseases program an immense and diverse audience for disseminating this project's interactive demonstrations and activities and in so doing, enhance the public understanding of science.