<qa>  What is the name of the study that is important now because of the increasing elderly population?Furthermore, studies on gastrointestinal illness in the elderly are important now because of an expanding elderly population and the heightened interest in the development of vaccine strategies. These developments necessitate a clearer understanding of the relative contribution of various pathogens to gastrointestinal illness in this population. Although many agents responsible for gastrointestinal illness have previously been difficult to identify reliably, state-of-the-art laboratory technology, including molecular methods, now make such study more feasible. We will use laboratory techniques to identify the specific causal agents of gastrointestinal illness in the intervention and placebo groups and to estimate the relative prevalence of these pathogens as a cause of gastrointestinal illness. 