After exposure to an enteropathogen, the manifestations of infectious diarrhea are variable and depend on host and pathogen factors. A variety of host factors modulate the likelihood of infection or severity of symptoms and can be categorized as those that mediate susceptibility (i.e., host genetic factors, pathogen receptors), and injury (i.e., stimulation of fluid and electrolyte channels, pro inflammatory cytokines). Resolution of infection is determined by factors that contribute to the phases of control and healing (i.e., anti-inflammatory cytokines and specific immunity). Our central hypothesis is that genetic polymorphisms that lead to qualitative or quantitative differences in one or several of these mediators are partially responsible for the development of infection and illness after exposure to enteric pathogens. To this end we will study two well-characterized populations of subjects with infectious diarrhea. The first study group will consist of healthy adults traveling from developed nations to areas of risk for infection with bacterial agents of diarrhea. The second study group will consist of healthy adults experimentally exposed to Cryptosporidium at the University of Texas - Houston Clinical Research Center. For both we propose to investigate host single nucleotide polymorphisms (SNPs) of genes that encode proteins that are associated with either susceptibility, modulation of disease manifestation (injury), eradication (control) and healing after infection. We will focus on three agents with potential for bioterrorism use by waterborne or food borne routes with distinct pathophysiology; enterotoxigenic E. coli a cause of secretory diarrhea, Enteroaggregative E. coli, a cause of inflammatory diarrhea and Cryptosporidium an intracellular pathogen. SNPs will be correlated with the isolation of an enteropathogen and clinical illness. The impact of SNPs will be examined in the context of different ethnic backgrounds. The understanding of the outcome of infection as they relate to host genetic factors will be of use in designing biodefense interventions that are directed towards improving risk assessment. The identification of populations that are more susceptible or vulnerable to the effects of enteric pathogens will be important in the design of strategies to decrease the impact that these agents may have in causing disease and defining the populations most likely to benefit from prevention, treatment and or vaccines.