<qa>  What is the secondary objective of stool specimens?A secondary objective is an estimate of the incidence of specific bacterial, viral, and protozoan agents in stool specimens collected from elderly individuals with gastrointestinal symptoms, and that might be related to water consumption. To determine the rates of asymptomatic infections, we will also test stool specimens from individuals who are not sick. Further questions about seroprevalence of these waterborne pathogens and the immune response they elicit will be answered by testing serum samples taken from all participants at critical junctures during the study. 